Low speed drives - the direct hydraulic drive approach. Brian Holmes of Hagglunds weighs up the advantages
When faced with a decision about a low speed drive forindustrial 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 lt/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 offer 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:-
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.
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 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 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 withvibrations 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 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 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.
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. 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. 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 maintenanceand 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 electro- mechanical drives.
Some weaknesses 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 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 e.g. 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. Somesuch applications are crushers and extruders.
Hydraulics have, in some cases, been associated with leakage, although gearboxes can also be a source of leakage 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 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.