LOAD SENSING

Understanding hydraulic load sensing control
Load sensing is a term used to describe a type of pump control employed in open circuits. It is so called because the load-induced pressure downstream of an orifice is sensed and pump flow adjusted to maintain a constant pressure drop (and therefore flow) across the orifice. The 'orifice' is usually a directional control valve with proportional flow characteristics, but a needle valve or even a fixed orifice can be used, depending on the application.
A load sensing circuit typically comprises a variable displacement pump, usually axial-piston design, fitted with a load sensing controller, and a directional control valve with an integral load-signal gallery (Exhibit 1). The load-signal gallery (LS, shown in red) is connected to the load-signal port (X) on the pump controller. The load-signal gallery in the directional control valve connects the A and B ports of each of the control valve sections through a series of shuttle valves. This ensures that the actuator with the highest load pressure is sensed and fed back to the pump.

To understand how the load-sensing pump and directional control valve function together in operation, consider a winch being driven through a manually actuated valve. The operator summons the winch by moving the spool in the directional valve 20% of its stroke. The winch drum turns at five rpm. For clarity, imagine that the directional valve is now a fixed orifice. Flow across an orifice decreases as the pressure drop across it decreases. As load on the winch increases, the load-induced pressure downstream of the orifice (directional valve) increases. This decreases the pressure drop across the orifice, which means flow across the orifice decreases and the winch slows down.
In a load sensing circuit the load-induced pressure downstream of the orifice (directional valve) is fed back to the pump via the load-signal gallery in the directional control valve. The load-sensing controller responds to the increase in load pressure by increasing pump displacement (flow) slightly so that pressure upstream of the orifice increases by a corresponding amount. This keeps the pressure drop across the orifice (directional valve) constant, which keeps flow constant and in this case, winch speed constant. The value of the pressure drop or delta P maintained across the orifice (directional valve) is typically 10 to 30 Bar (145 to 435 PSI). When all spools are in the center position the load-signal port is vented to tank and the pump maintains 'standby' pressure equal to or slightly higher than the load sensing controller's delta P setting.
Because the pump always receives the load signal from the function operating at the highest pressure, high-end load sensing directional control valves feature a pressure compensator (not shown) at the pressure inlet to each section. The section pressure compensator works with the spool-selected orifice opening to maintain a constant flow, independent of the pressure variations caused by the operation of multiple functions at the same time. This is sometimes referred to as 'sensitive load sensing'.
A load sensing pump only produces the flow demanded by the actuators - this makes it energy efficient (fewer losses to heat) and as demonstrated in the above example, provides more precise control. Load-sensing control also provides constant flow independent of pump shaft speed variations. If pump drive speed decreases, the load-sensing controller will increase displacement (flow) to maintain the set delta P across the directional control valve (orifice), until maximum displacement is reached.
Load sensing pump controls usually incorporate a pressure limiting control, also referred to as a pressure cut-off or pressure compensator. The pressure compensator limits maximum operating pressure by reducing pump displacement to zero when the set pressure is reached

Power saving with hydraulic load sensing control
Load sensing is a term used to describe a type of variable pump control used in open circuits. It is so called because the load-induced pressure downstream of an orifice is sensed and pump flow is adjusted to maintain a constant pressure drop (and therefore flow) across the orifice. The 'orifice' is usually a directional control valve with proportional flow characteristics, but a needle valve or even a fixed orifice can be employed, depending on the application.
In hydraulic systems that are subject to wide fluctuations in flow and pressure, load-sensing circuits can save substantial amounts of input power. This is illustrated in Exhibit 1. In systems where all available flow (Q) is continuously converted to useful work, the amount of input power lost to heat is limited to inherent inefficiencies. In systems fitted with fixed displacement pumps where 100 percent of available flow is only required intermittently, the flow not required passes over the system relief valve and is converted to heat. This situation is compounded if the load-induced pressure (p) is less than the set relief pressure - resulting in additional power loss due to pressure drop across the metering orifice (control valve).

Exhibit 1. Flow-pressure-power diagrams for fixed, variable and load sensing controlled hydraulic pumps (Peter Rohner).
A similar situation occurs in systems fitted with pressure controlled (pressure compensated) variable pumps, when only a portion of available flow is required at less than maximum system pressure. Because this type of control regulates pump flow at the maximum pressure setting, power is lost to heat due to the potentially large pressure drop across the metering orifice.
A load sensing controlled variable pump largely eliminates these inefficiencies. The power lost to heat is limited to the relatively small pressure drop across the metering orifice, which is held constant across the system's operating pressure range (see bottom of Exhibit 1).
A load sensing circuit typically comprises a variable displacement pump, usually axial-piston design, fitted with a load sensing controller, and a directional control valve with an integral load-signal gallery (Exhibit 2). The load-signal gallery (LS, shown in red) is connected to the load-signal port (X) on the pump controller. The load-signal gallery in the directional control valve connects the A and B ports of each of the control valve sections through a series of shuttle valves. This ensures that the actuator with the highest load pressure is sensed and fed back to the pump control.

Exhibit 2. Typical load sensing circuit

To understand how the load-sensing pump and directional control valve function together in operation, consider a winch being driven through a manually actuated valve. The operator summons the winch by moving the spool in the directional valve 20 percent of its stroke. The winch drum turns at five rpm. For clarity, imagine that the directional valve is now a fixed orifice. Flow across an orifice decreases as the pressure drop across it decreases. As load on the winch increases, the load-induced pressure downstream of the orifice (directional valve) increases. This decreases the pressure drop across the orifice, which means flow across the orifice decreases and the winch slows down.
In a load sensing circuit the load-induced pressure downstream of the orifice (directional valve) is fed back to the pump control via the load-signal gallery in the directional control valve. The load-sensing controller responds to the increase in load pressure by increasing pump displacement (flow) slightly so that pressure upstream of the orifice increases by a corresponding amount. This keeps the pressure drop across the orifice (directional valve) constant, which keeps flow constant and in this case, winch speed constant. The value of the pressure drop or delta p maintained across the orifice (directional valve) is typically 10 to 30 Bar (145 to 435 PSI). When all spools are in the center or neutral position the load-signal port is vented to tank and the pump maintains 'standby' pressure equal to or slightly higher than the load sensing control's delta p setting.
Because the variable pump only produces the flow demanded by the actuators, load-sensing control is energy efficient (fewer losses to heat) and as demonstrated in the above example, improves actuator control. Load-sensing control also provides constant flow independent of pump shaft speed variations. If pump drive speed decreases, the load-sensing controller will increase displacement (flow) to maintain the set delta p across the directional control valve (orifice), until displacement is at maximum. To further your knowledge on load sensing and other variable pump controls,

FLUSHING

Flushing hydraulic systems
Techniques for flushing hydraulic systems vary in cost and complexity. Before I discuss some of these methods, let's first distinguish between flushing the fluid and flushing the system.
The objective of flushing the fluid is to eliminate contaminants such as particles and water from the fluid. This is usually accomplished using a filter cart or by diverting system flow through an external fluid-conditioning rig.
The objective of flushing the system is to eliminate sludge, varnish, debris and contaminated or degraded fluid from conductor walls and other internal surfaces, and system dead spots. Reasons for performing a system flush include:
Fluid degradation - resulting in sludge, varnish or microbial deposits.
Major failure - combined with filter overload disperses debris throughout the system.
New or overhauled equipment - to purge 'built-in' debris.
Common methods for flushing hydraulic systems include:
Double oil and filter change.
Mechanical cleaning.
Power flushing.
The technique or combination of techniques employed will depend on the type of system and its size, reliability objectives for the equipment and the reason for the flush.
Double oil and filter change
This technique involves an initial oil drain and filter change, which expells a large percentage of contaminants and degraded fluid. The system is then filled to the minimum level required and the fluid circulated until operating temperature is reached and the fluid has been turned over at least five times. The oil is drained and the filters changed a second time. An appropriate oil analysis test should be performed to determine the success of the flush. To maximize the effectiveness of this technique, the system should be drained as thoroughly as possible and the reservoir mechanically cleaned.
Mechanical cleaning
Although not technically a flushing technique, the selective use of mechanical cleaning may be incorporated in the flushing strategy. This can involve the use of a pneumatic projectile gun to clean pipes, tubes and hoses (see exhibit 1), and disassembly of the reservoir and other components for cleaning using brushes and solvents. Mechanical cleaning is labor intensive and therefore costly. It carries with it reliability risks associated with opening the hydraulic system and intervention by human agents.

Exibit-1

Power flushing
Power flushing involves the use of a purpose-built rig to circulate a low viscosity fluid at high velocities to create turbulent flow conditions (Reynolds number > 2000). The flushing rig is typically equipped with a pump that has a flow rate several times that of system's normal flow, directional valves, accumulators, fluid heater and chiller and of course, a bank of filters. The directional valves enable the flushing direction to be changed, the accumulators enable pulsating flow conditions and the heater and chiller enable the fluid temperature to be increased or decreased, all of which can assist in the dislodgment of contaminants. Analysis of the flushing fluid is performed regularly during the flushing operation to determine the point at which the system has been satisfactorily cleaned.
What about components?
The question of how to deal with system components arises when contemplating a system flush. Plumbing should be flushed first in isolation from pumps, valves and actuators. Once the conductors have been flushed clean, valves and actuators can be gradually included in the flushing circuit. The decision to disassemble and mechanically clean components will depend on the type of equipment, your reliability objectives and the reason for the flush.
Prevent or cure?
With the exception of new or overhauled equipment, the need to flush a hydraulic system generally represents a failure of maintenance.

REDUCING NOISE EMESSION

Reducing noise emission from hydraulic systems
Many industrialized countries have regulations that restrict noise levels in the workplace. The high power density, and corresponding high noise emission of hydraulic components means that industrial hydraulic systems are often the target of efforts to reduce mean noise levels in the workplace.
The dominant source of noise in hydraulic systems is the pump. The hydraulic pump transmits structure-borne and fluid-borne noise into the system and radiates air-borne noise.
All positive-displacement hydraulic pumps have a specific number of pumping chambers, which operate in a continuous cycle of opening to be filled (inlet), closing to prevent back flow, opening to expel contents (outlet) and closing to prevent back flow.
These separate but superimposed flows result in a pulsating delivery, which causes a corresponding sequence of pressure pulsations. These pulsations create fluid-borne noise, which causes all downstream components to vibrate. The pump also creates structure-borne noise by exciting vibration in any component with which it is mechanically linked, e.g. tank lid. The transfer of fluid and structure induced vibration to the adjacent air mass results in air-borne noise.
Reducing fluid-borne noise
While fluid-borne noise attributable to pressure pulsation can be minimized through hydraulic pump design, it cannot be completely eliminated. In large hydraulic systems or noise-sensitive applications, the propagation of fluid-borne noise can be reduced by the installation of a silencer. The simplest type of silencer used in hydraulic applications is the reflection silencer, which eliminates sound waves by superimposing a second sound wave of the same amplitude and frequency at a 180-degree phase angle to the first.
Reducing structure-borne noise
The propagation of structure-borne noise created by the vibrating mass of the power unit (the hydraulic pump and its prime mover) can be minimized through the elimination of sound bridges between the power unit and tank, and the power unit and valves. This is normally achieved through the use of flexible connections i.e. rubber mounting blocks and flexible hoses, but in some situations it is necessary to introduce additional mass, the inertia of which reduces the transmission of vibration at bridging points.
Reducing air-borne noise
The magnitude of noise radiation from an object is proportional to its area and inversely proportional to its mass. Reducing an object's surface area or increasing its mass can therefore reduce its noise radiation. For example, constructing the hydraulic reservoir from thicker plate (increases mass) will reduce its noise radiation.
The magnitude of air-borne noise radiated directly from the hydraulic pump can be reduced by mounting the pump inside the tank. For full effectiveness, there must be a clearance of 0.5 meters between the pump and the sides of tank, and the mounting arrangement must incorporate decoupling between the power unit and tank to insulate against structure-borne noise. The obvious disadvantage of mounting the hydraulic pump inside the tank is that it restricts access for maintenance and adjustment.
If hydraulic system noise remains outside the required level after all of the above noise propagation countermeasures have been exhausted, encapsulation or screening must be considered.

TEMPERATURE SHOCK

What is temperature shock?
When there is a significant difference between the temperature of a hydraulic component and the fluid being supplied to it, rapid, localized heating of the internal parts of the component can occur. This causes individual parts of the component to expand at different rates, resulting in interference between parts that normally have fine clearances.
How does this happen?
Temperature shock occurs when part of a hydraulic circuit is operated for long enough for the hydraulic fluid in the system to reach operating temperature, and then a previously idle part of the circuit is functioned. This results in hot fluid being delivered to cold components.
How can this be prevented?
To prevent temperature shock of hydraulic motors, the motor's case must be continuously 'flushed' (positive circulation of a relatively small volume of fluid through the case). This ensures that the motor is always at the same temperature as the fluid in the system.

DIESEL EFFECT

• What is the 'diesel effect'?


• The diesel effect occurs in a hydraulic cylinder when air is drawn past the rod seals, mixes with the hydraulic fluid and explodes when pressurized.
  How does this affect a hydraulic cylinder?


• When a double-acting hydraulic cylinder retracts under the weight of its load, the volume of fluid being demanded by the rod side of the cylinder can exceed the volume of fluid being supplied by the pump.
  When this happens, a negative pressure develops in the rod side of the hydraulic cylinder, which usually results in air being drawn into the cylinder past its rod seals. This occurs because most rod seals are designed keep high-pressure fluid in and are not designed to keep air out. The result of this is aeration - the mixing of air with the hydraulic fluid.
  Aeration causes damage through loss of lubrication and overheating, and when a mixture of air and oil is compressed it can explode, damaging the hydraulic cylinder and burning its seals. As you have probably gathered, the term 'diesel effect' is a reference to the combustion process in a diesel engine.
  cavator is to allow the boom or arm to be lowered rapidly under its own weight.
  When activated, this valve connects the ports of the hydraulic cylinder together allowing it to retract under the weight of the boom or arm. The fluid displaced from the piston side of the cylinder is directed with priority to the rod side, before any excess volume is returned to the hydraulic reservoir. An orifice controls the speed with which the hydraulic cylinder retracts.
  If this valve malfunctions or is set incorrectly, a negative pressure can develop on the rod side of the hydraulic cylinder, causing air to be drawn past the rod seals, leading to failure of the cylinder.
  How can this type of failure be prevented?


• By checking the operation and adjustment of circuit protection devices at regular intervals. As in this case, if the faulty float valve had been identified early enough, the failure of this hydraulic cylinder and the significant expense of its repair could have been prevented.

HYDRAULIC FILTERS

Hydraulic filters that do more harm than good

Given that particle contamination of hydraulic fluid reduces the service life of hydraulic components, it would seem logical that a system can never have too many hydraulic filters. Well... not exactly.

Some hydraulic filters can actually do more harm than good and therefore their inclusion in a hydraulic system is sometimes misguided.

Pump inlet (suction) filters fall into this category. Inlet filters usually take the form of a 140 micron, mesh strainer which is screwed onto the pump intake penetration inside the hydraulic reservoir.

Inlet filters increase the chances of cavitation occurring in the intake line and subsequent damage to, and failure of the hydraulic pump. Piston-type pumps are particularly susceptible.

If the reservoir starts out clean and all fluid returning to the reservoir is filtered, inlet filters are not required since the hydraulic fluid will not contain particles large enough to be captured by a coarse mesh strainer.

What does this mean?
I generally recommend removing and discarding inlet filters where fitted. The one possible exception to this rule is charge pump intakes on hydrostatic transmissions. If in doubt consult the hydraulic pump manufacturer.

If you are involved in the design of hydraulic systems, think twice before fitting hydraulic filters to pump intake lines.
"The one thing a suction strainer does that's worthwhile is to keep out the trash that gets dropped into the tank during service. We lost pumps to things like bolts that we know were not in the tank when it got built. The process of adding hydraulic fluid to the tank often doubles as the trash-installation function. The screens that are often installed in the fill neck usually get a hole poked through them so that oil will go in faster..."

HYDRAULIC FAILURES

Hydraulic pump life cut short by particle contamination

What is 'contaminated hydraulic fluid'?

Contaminants of hydraulic fluid include solid particles, air, water or any other matter that impairs the function of the fluid.

How does contamination affect a hydraulic pump?

Particle contamination accelerates wear of hydraulic components. The rate at which damage occurs is dependent on the internal clearance of the components within the system, the size and quantity of particles present in the fluid, and system pressure.

Particles larger than the component's internal clearances are not necessarily dangerous. Particles the same size as the internal clearances cause damage through friction. However, the most dangerous particles in the long term are those that are smaller than the component's internal clearances.

Particles smaller than 5 microns are highly abrasive. If present in sufficient quantities, these invisible 'silt' particles cause rapid wear, destroying hydraulic pumps and other components

How can this type of hydraulic pump failure be prevented?

While the type of failure described above is unusual in properly designed hydraulic systems that are correctly maintained, this example highlights the importance of monitoring hydraulic fluid cleanliness levels at regular intervals.

As in this case, if the high levels of silt particles present in the hydraulic fluid had been identified and the problem rectified early enough, the damage to this hydraulic pump and the significant expense of its repair could have been avoided


Hydraulic fluid - getting the viscosity right
Most hydraulic systems will operate satisfactorily using a variety of fluids, including multi-grade engine oil and automatic transmission fluid (ATF), in addition to the more conventional anti-wear (AW) hydraulic fluid - provided the viscosity is correct.

Viscosity is the single most important factor when selecting a hydraulic fluid. It doesn't matter how good the anti-wear, anti-oxidization or anti-corrosion properties of the fluid are, if the viscosity grade is not correctly matched to the operating temperature range of the hydraulic system, maximum component life will not be achieved.

Defining the correct fluid viscosity grade for a particular hydraulic system involves consideration of several interdependent variables. These are:

starting viscosity at minimum ambient temperature;
maximum expected operating temperature, which is influenced by maximum ambient temperature; and
permissible and optimum viscosity range for the system's components.

Once these parameters are known, the correct viscosity grade can be determined using the viscosity/temperature curve of a suitable type of fluid - commonly AW hydraulic fluid defined according to ISO viscosity grade (VG) numbers.

Automatic transmission fluid, multi-grade engine oil and anti-wear, high VI (AWH) hydraulic fluid are commonly used in hydraulic systems that experience a wide operating temperature range. These fluids have a higher Viscosity Index (VI) than AW hydraulic fluids due to the addition of VI improvers. The higher the VI a fluid has, the smaller the variation in viscosity as temperature changes.

In simple terms, this means that if you are running ATF(46) in your skid-steer loader, you can operate the hydraulics with a higher fluid temperature before viscosity falls below optimum, than you could if you were running ISO VG46 AW hydraulic fluid.

When selecting a high VI fluid, the component manufacturer's minimum permissible viscosity value should be increased by 30% to compensate for possible loss of viscosity as a result of VI improver sheardown.

VI improvers can have a negative effect on the demulsification and air separation properties of the fluid and for this reason some hydraulic component manufacturers recommend that these types of fluids only be used when operating conditions demand.

As far as fluid recommendations go, for commercial reasons relating to warranty etc, I always advise following the machine manufacturer's recommendation. But in equipment that has a history of satisfactory performance and component life, there is usually no compelling reason to change the type of fluid being used.

High hydraulic fluid temperature - how it causes premature failures
I was asked recently to conduct failure analysis on two radial piston hydraulic motors that had failed well short of their expected service life. Inspection revealed that the motors had failed through inadequate lubrication, as a result of low fluid viscosity caused by excessive hydraulic fluid temperature.

How does this happen?
As the temperature of petroleum-based hydraulic fluid increases, its viscosity decreases. If fluid temperature increases to the point where viscosity falls below the level required to maintain a lubricating film between the internal parts of the component, damage will result.

The temperature at which this occurs depends on the viscosity grade of the fluid in the system. Hydraulic fluid temperatures above 180°F (82°C) damage seals and reduce the service life of the fluid. But depending on the grade of fluid, viscosity can fall to critical levels well below this temperature.

How can this type of failure be prevented?
The above example highlights the importance of not allowing fluid temperature to exceed the point at which viscosity falls below the optimum level for the system's components.

Continuing to operate a hydraulic system when the fluid is over-temperature is similar to operating an internal-combustion engine with high coolant temperature. Damage is pretty much guaranteed.

Therefore, whenever a hydraulic system starts to overheat, shut down the system, find the cause of the problem and fix it!

What is cavitation?
Cavitation occurs when the volume of hydraulic fluid demanded by any part of a hydraulic circuit exceeds the volume of fluid being supplied.

This causes the absolute pressure in that part of the circuit to fall below the vapor pressure of the hydraulic fluid. This results in the formation of vapor bubbles within the fluid, which implode when compressed.

Cavitation causes metal erosion, which damages hydraulic components and contaminates the hydraulic fluid. In extreme cases, cavitation can result in major mechanical failure of pumps and motors.

While cavitation commonly occurs in the hydraulic pump, it can occur just about anywhere within a hydraulic circuit.


What is the 'diesel effect'?
The diesel effect occurs in a hydraulic cylinder when air is drawn past the rod seals, mixes with the hydraulic fluid and explodes when pressurized.

How does this affect a hydraulic cylinder?
When a double-acting hydraulic cylinder retracts under the weight of its load, the volume of fluid being demanded by the rod side of the cylinder can exceed the volume of fluid being supplied by the pump.

When this happens, a negative pressure develops in the rod side of the hydraulic cylinder, which usually results in air being drawn into the cylinder past its rod seals. This occurs because most rod seals are designed keep high-pressure fluid in and are not designed to keep air out. The result of this is aeration - the mixing of air with the hydraulic fluid.

Aeration causes damage through loss of lubrication and overheating, and when a mixture of air and oil is compressed it can explode, damaging the hydraulic cylinder and burning its seals. As you have probably gathered, the term 'diesel effect' is a reference to the combustion process in a diesel engine.

SIX MISTAKES

Mistake #1 Changing the oil

There's only two conditions that necessitate a hydraulic oil change. If you're changing your oil based on an arbitrary number of hours in service or when it's contaminated with particles or water, you're pouring money down the drain.

Mistake #2 Changing the filters

Same goes for hydraulic filters. If you're changing them on hours, you're either changing them too early or too late. Too early and you're only wasting money on unnecessary filter changes. Too late and you're quietly reducing the service life of every component in the hydraulic system - costing you much more in the long run.

Mistake #3 Running too hot

Would you continue to operate an engine that was overheating? Didn't think so. And like an engine, the fastest way to destroy hydraulic components, seals, hoses and the oil itself, is high-temperature operation. But do you know the operating temperature above which you're doing irreversible damage to your hydraulic equipment? Not knowing this one number could cost you a small fortune.

Mistake #4 Using the wrong oil

The oil is THE most important component of any hydraulic system. And buying it on price alone is brain-dead stupid. But there's one property of hydraulic oil - above all others, that's crucial for optimum machine performance and service life. And despite what you might think, you won't always get this right by blindly following the machine manufacturer's recommendation. Get it wrong however, and you're the one who'll pay dearly.

Mistake #5 Wrong filter locations

Any filter is a good filter, right? Wrong! There's two hydraulic filter locations that can rapidly destroy the very components they were installed to protect. If these filters are fitted to your machine and you don't get rid of 'em-and fast, they'll end up doing serious damage to major components - and your bank account. Oh, and if you think the manufacturer of your hydraulic machine wouldn't have been dumb enough to install these filters in the first place - you might be in for shock.

Mistake #6 Believing hydraulic components are self-priming and self-lubricating

Would you start an engine with no oil in the crankcase? Of course not. And yet if you, or the mechanics who work for you, believe that because oil circulates through hydraulic components in operation, they don't require any special attention during installation - this amounts to the same thing. Oh sure, the component may work OK for a while, but the damage done at start-up dooms it to premature failure. Not commissioning hydraulic components correctly can cost you thousands of dollars in lost service life.

HYDRAULIC LEAKS

If you're serious about eliminating hydraulic leaks,
then the scourge of your hydraulic plumbing is
the tapered thread connection - NPT or BSPT. I'm a big advocate of eliminating tapered threads
from your hydraulic plumbing - but I also understand
that sometimes, what's ideal is not always feasible.

When they can't be 'engineered out', this is
how I deal with them:

First, don't waste your time with thread tape -
I only ever use it when there's no thread sealing
compound available. I've had best success with
Loctite 567 and 577 (my favorite), which are pastes
rather than liquids like some of the others.

If you're re-sealing a joint, thorough cleaning
of the old adaptor and port is essential. If you're
in your workshop, a brush wheel in a bench grinder
does a marvellous job on the threads of the adaptor.
But the female threads in the port aren't so easy.

Once you've removed all remnants of thread tape
or sealant, the next step is to use the
appropriate Loctite cleaner.
Don't skip this step - you'll regret it.

Next, if you're not able to wait the 6 hours
or so for acceptable cure strength, apply the
Activator 7649 and allow it to dry. If you have
the luxury of leaving the joint overnight
before pressurizing it, you can skip this step.

Starting two threads back, apply a bead of paste
around the entire circumference - completely
filling the threads. Do the same with
the female threads in the port. Don't be stingy -
it's better to waste some product than under fill
the threads and have the joint leak.

Now torque the joint. And don't be afraid to
swing on it either. For larger diameter adaptors,
I use a 3/4" drive socket with a long bar.
This part of the instruction is not very scientific,
but hey, there's nothing scientific about taper threads.

For best results, wait as long as possible before
pressurizing the joint - but at least half an hour -
even when using the Activator.

That's all there is to it. But still no guarantees of course.
Which is why it's always better to get rid of 'em if you can.

PNUMATIC CAR

A couple of month's back we talked about the imminent
launch of the pneumatic car - billed by it's makers
as a zero pollution vehicle. The first commercial compressed air car is on the
verge of production in a recently signed partnership
with Tata Motors, India's largest automotive manufacturer.

The MiniC.A.T is a light urban car, with a tubular chassis
that is glued not welded and a body of fiberglass.
The revolutionary electrical system uses just one cable
and the vehicle has a wireless control system.
There are no keys - just an access card.

Ninety cubic meters of compressed air is stored
in fiber tanks. The expansion of this air drives
the pistons in the specially designed motor
to propel the vehicle.

The air conditioning system makes use of the expelled
cold air. Due to the absence of combustion and
resultant residues, an oil change (1 liter of vegetable oil)
is only necessary every 30,000 miles.

According to the designers, it costs less than one Euro
per 100 kilometers (about a tenth of a petrol car).
Its mileage is about double that of the most advanced
electric car (200 to 300 kilometers or 10 hours of driving).

Refilling the car will, take place at petrol stations adapted
to supply compressed air. In two or three minutes, and at
a cost of approximately 1.5 Euros, the car will be ready to go
another 200-300 kilometers.

Could this be global warming spokesman Al Gore's dream machine?
Some of us are skeptical. Here's a downbeat assessment
from one of our member's:

"Compressed air vehicles are not merely hype - they are a scam.
I have used compressed air extensively and it's invaluable in
a few specialized applications. It has dreadful energy efficiency
over the full cycle; somewhere in single figure percent.
If one is bent on increasing global warming and raising the cost
of driving, then widespread adoption of pneumatic vehicles
is the way to go."

I have to add, speaking as a keen scuba diver, the most
innovative use of compressed air developed in the last century
was the aqualung, invented by Jacques Cousteau.

Seriously though, while the pneumatic car itself may have
acceptable efficiency - the air has to be compressed
in the first place. And this is not an energy efficient exercise.

What's more, if all these pneumatic cars leak air like your
average industrial pneumatics installation, then this will
blow the cars' running cost out of the water. It'd be like
driving your conventional car with a hole in the petrol tank.

As a matter of fact, I'm thinking about approaching
Tata Motors with the idea of including a copy of
Peter Rohner's pneumatics book in the glove compartment
of every pneumatic car they produce. Because it explains
what you must know if you want to minimize the operating
cost of your pneumatic equipment.

Of course, you don't have to be the proud owner of
a pneumatic car to justify having a copy. In fact,
if you have anything at all to do with pneumatic equipment
it's essential reading: