The direct hydraulic drive approach to low speeds

Faced with a decision about a low speed drive for industrial plant, most engineers will tend to follow the traditional line of thinking: but there is an alternative using direct hydraulic drive

When faced with a decision about a low speed drive for industrial plant and equipment, most engineers will tend to follow the traditional line of thinking which will comprise of some kind of high speed motor or turbine and a gearbox selected to reduce the speed for the duty required.

This solution will often require other elements in the drive train to complete the installation such as couplings, clutches, pulleys, belts etc: and if foundations are required which is very often the case on large installations, costly civil works can also be needed.

Sometimes these drive arrangements are very complex with quite high design cost.

Except for high speed applications, electric drives will usually need some kind of speed reduction to achieve the needed speed and torque range.

Engineers should be aware that there is an alternative using direct hydraulic drive.

Hydraulic motors have been used to drive all kinds of plant and machines for a long time.

The excellent controllability, flexibility and user friendly nature in use with an unbeatable power to weight ratio were and still are strong attractive reasons for using fluid power.

The traditional applications for hydraulic motors tended to be the mobile and marine markets where volume was available to justify the development of the technology.

Such applications as wheel or track drives and winches were the main types but these applications often used gearboxes and are also intermittent where life issues were not so important and motor displacements were available up to around 12 to 15 lt/rev but mainly below 8 lt/rev.

Hydraulic motors have developed and there are now a very wide range of direct hydraulic drives available up to a massive 250 lt/rev and over 1MW in power from a single unit completely eliminating the need of gear boxes on low speed plant.

The great advantages of hydraulics, with the inherent simplicity and flexibility can now be applied to a range of heavy duty industrial applications which operate 24/7, and where efficiency, reliability and long life are prerequisites.

These drives are available for speeds from zero to 300 rpm and used on small agitators right up to huge bulk handling plant and processing applications.

Full Control of Power, Torque and Speed.

Hydraulic drives use variable displacement axial piston pumps to supply variable flow to the drive motors thereby providing variable speed which is essential for efficient processes.

They also provide the means to easily control all three basic drive factors.

By controlling pressure, we directly control the torque and by controlling the flow of oil to the hydraulic motor, we control the speed.

Both can be precisely controlled and integrated into power control by electrical signals.

Thus complete control of the drive is available bringing high functionality and versatility to the drive.

Machines become not just slaves to the operator but intelligent production units which react to situations, prevent bottlenecks and increase production.

Some characteristics of hydraulic drives, (In particular radial piston cam curve design, for example the Hagglunds Drives CB range).

* Low inertia.

* Full torque available from zero to maximum speed.

* Shockproof Perfect load sharing.

* Can be applied in almost any environment.

* High power to weight.

* Compact and weight saving.

* No problems with EMC or distortions on the power supplies.

Some typical applications are: Steel plate conveyors and feeders, belt conveyors, bucket wheel machines, shredders, mills, mixers, winches, drilling and injection moulding machines.

The hydraulic drive has very low inertia and so can react instantly giving excellent response and control.

For example roll mills can stop instantly compared to high inertia gearbox drives which take longer to slow down.

This is a significant improvement in the safety of operators working for example in the rubber and plastics industry.

On drives with high speed motor and gearbox, a very high moment of inertia is evident and if the machine is liable to shocks for example on a shredder or crusher when a hard object enters the transient forces experienced can often damage the machine and the drive, causing costly repair and time in lost production.

The direct hydraulic drive in the same circumstances stops instantly, the forces limited by fast acting pressure controls, are kept within design limits and so no damage and less wear is caused and the drive can go back into production immediately.

This process enables high reliability on heavy duty plant where costly breakdown and high maintenance costs were accepted in the past as a natural consequence of this type of machine.

Of course in hydraulic drives, the whole system is charged with oil, cushioned so to speak and can therefore be applied on tough applications, with vibrations and shock, without problems.

With electro-mechanical drives, the torque at low speed is often time restricted or can be reduced significantly.

With a Hagglunds motor, the mechanical efficiency of the motor is very high.

You do not have to compromise the machine function - whatever speed you require, you always know you have full driving force available.

On many types of machine this can mean the difference between starting up without problems or not being able to start and facing time and effort to bring the machine into an unloaded condition to enable start; resulting in hours of lost production.

It can also mean that in some cases in an attempt to eliminate this possibility, the electro-mechanical drive is oversized with more cost, less efficiency and higher running costs.

Drive direction is also no problem for hydraulics; the drive can be reversed instantaneously and smoothly with a simple electric signal to the piston pump.

This is very useful on applications like shredders where jamming can occur and where reversing, particularly with full torque at zero speed can unjam machines and keep production flowing.

These characteristics are also important on constant tension systems where the drive has to maintain a constant tension on a wire rope using a winch either paying out or pulling in or indeed holding steady.

Hydraulic drives by their very nature have perfect load sharing characteristics.

If two motors are driving a drum for example, the pressure from the common pump unit would naturally provide equal pressure to the drive motors which would effectively share the load.

Electro-mechanical drives would either have to be mechanically linked or provide some form of electronic synchronisation which can lead to problems and vibrations.

Conventional gear train drives can take up a lot of space around the machine and by definition they must be mechanically connected and properly aligned.

This can get in the way of operation and maintenance of the machine and if belts and chains are involved bulky guarding etc is needed to ensure safety.

Direct hydraulic drives mount directly on the drive shaft or drum of the machine.

There are no foundation requirements and no alignment problems.

The power unit is positioned out of the way with just the hydraulic pipework connection to take care of.

The whole machine look and concept can be transformed improving machine performance, reducing weight, improving operator access and reducing maintenance.

The drive can be adjusted and monitored away from the machine (remotely by modem too) improving safety for all staff and noise levels are lower, well inside normal regulations.

The hydraulic motor is filled with oil and this is drained to tank.

If the environmental conditions are hot or cold, a flow of oil can be used to either cool or warm the motor to protect from those extremes and the power unit can obviously be provided with heaters and coolers as required.

The flat plate distributor in the motor is not sensitive to temperature variation and the rugged design can be exposed to dusty, humid and even underwater conditions.

For explosion proof areas, the standard hydraulic motor is normally acceptable and there is zero maintenance on the motor itself, so it can be designed into areas that have little or restricted access.

The electric motors used with hydraulic drives are standard squirrel cage induction motors running efficiently at constant speed.

The pumps are controlled by a simple electrical signal to a control card.

Therefore no problems are introduced as regards EMC or distortions on the power supplies which has been a major and costly concern with AC variable speed electro-mechanical drives.

Some weakness of hydraulic drives.

The hydraulic design is heavy duty as standard therefore it may not be competitive on price if applied on low power, light duty, fixed speed applications unless there are other considerations such as the environment to consider.

On some applications high inertia is a benefit, for example on crushers of very hard materials where it would be uneconomic to produce the torque needed to shear the material with a hydraulic motor.

Where exact synchronisation of drives with varying loads and speeds is required hydraulic drives do not offer the best solution due to the difficulty of controlling the fluid under all circumstances.

This type of installation suits a high inertia drive.

For example steel rolling mills.

Where the gearbox has to be designed into the very machine it is driving, so utilising the shear bulk of the gearbox obviously precludes the use of a direct hydraulic drive without extensive redesign.

Some such applications are crushers and extruders.

Hydraulics has in some cases been associated with leakage, although gearboxes can also be a source of leakage.

Even with big improvements in recent years this issue is still a concern although with good practice and regular attention leakage can be eliminated or at least reduced to a satisfactory level.

To summarise, the direct hydraulic drive provides a variable low speed drive with some unique features and when properly applied gives a very reliable system with real competitive advantages.

They can be up rated and adjusted to changing requirements fairly easily.

On highly critical applications for example on cement kilns or chemical reactors the hydraulic system can be arranged to provide redundancy in the event of a catastrophic failure of a pump so eliminating the chance of stopping the machine.

Several drives and auxiliaries can also be supplied from a single power unit which can add flexibility to the options available.

The layout, type and permutations of power unit are endless but companies like Hagglunds Drives tend to use a modular system with standard components and controls which are well matched specifically for drives and can take full responsibility for the complete drive which adds a security advantage for the end user.

The hydraulic motors and pumps are all standard items and therefore available on short lead times, unlike some large gearboxes which can take many months to acquire.

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