Hydraulic drive systems offer a simple alternative

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

When faced with a decision about a low speed drive for industrial plant and equipment, there is a tendency to follow the traditional line of thinking of some kind of high speed motor or turbine and a gearbox selected for the speed and torque required.

This solution will often require other elements in the drive train to complete the installation such as couplings, clutches, pulleys, belts etc and foundations.

Sometimes these arrangements are complex with quite high design cost.

However, there is an attractive and simple alternative of 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.

But hydraulic motors have developed and there are now a very wide range of direct hydraulic drives available up to a massive 250 litre/rev and over 1MW in power from a single unit, completely eliminating the need of gearboxes.

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

Hydraulics offers full control of power, torque and speed by using axial piston pumps supplying variable flow to the drive motors thereby providing variable speed which is essential for efficient processes.

And by controlling pressure, we directly control the torque.

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.

Some characteristics of hydraulic drives include: 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; and no problems with EMC or distortions on the power supplies.

Typical applications include steel plate feeders, belt conveyors, bucket wheel reclaimers, 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 with 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 crusher when an uncrushable 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 giving high reliability and low maintenance.

Hydraulic drives are charged with oil, cushioned so to speak and can therefore be applied on tough applications with vibrations and shock without problems.

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

With a Hagglunds motor, for example, the mechanical efficiency of the motor is very high and high torque can be maintained indefinitely even when at stalling pressure and speed.

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 resulting in hours of lost production.

It can also mean that in some cases the electromechanical 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.

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 effectively sharing the load.

Electromechanical 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.

Direct hydraulic drives mount directly on the drive shaft 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 and noise levels are lower, well inside normal regulations.

If the environmental conditions are hot or cold, 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 rugged design can be exposed to dusty, humid and even underwater conditions.

For explosion proof areas, the standard hydraulic motor is normally acceptable.

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 electromechanical drives.

However, hydraulic drives do have some weaknesses.

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 multiple drives or the environment to consider.

On some applications high inertia is a benefit, for example: on crushers of very hard materials; where exact synchronisation of drives with varying loads and speeds is required under all circumstances, eg steel rolling mills; or where the gearbox has to be designed into the very machine it is driving, so using the shear bulk of the gearbox.

Some such applications are crushers and extruders.

Hydraulics have in some cases been associated with leakage.

Although gearboxes can also be a source of leakage, with good practise 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 easily be up rated and adjusted to changing requirements.

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

Several drives and auxiliaries can be supplied from a single power unit adding 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.

Back to top