SB Aug42 drives.qxd 8/8/07 11:00 Page 44
Drives
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
The damaging flywheel effect - of gearbox drives
speeds is required under all circumstances e.g. steel rolling mills.
Where the gearbox has to be designed into the very machine
With electro-mechanical drives, the torque at low speed is it is driving, so utilising the shear bulk of the gearbox. Some
often time restricted or can be reduced significantly. With a such applications are crushers and extruders.
Hagglunds motor for example, the mechanical efficiency of the Hydraulics have, in some cases, been associated with leakage,
motor is very high and high torque can be maintained indefinitely although gearboxes can also be a source of leakage although
even when at stalling pressure and speed. You do not have to with good practice and regular attention leakage can be
compromise the machine function - whatever speed you require, eliminated or at least reduced to a satisfactory level.
you always know you have full driving force available. On many To summarise, the direct hydraulic drive provides a variable
types of machine this can mean the difference between starting low speed drive with some unique features and when properly
up without problems or not being able to start resulting in hours applied gives a very reliable system with real competitive
of lost production. It can also mean that in some cases the advantages. They can easily be up rated and adjusted to
electro-mechanical drive is oversized with more cost, less changing requirements. On critical applications for example on
efficiency and higher running costs. cement kilns or chemical reactors the system can provide
Drive direction is also no problem for hydraulics; the drive can redundancy in the event of a failure of a pump so eliminating the
be reversed instantaneously and smoothly with a simple electric chance of stopping the machine.
signal to the piston pump. This is very useful on applications like Several drives and auxiliaries can be supplied from a single
shredders where jamming can occur and where reversing, power unit adding flexibility to the options available. The layout,
particularly with full torque at zero speed can unjam machines type and permutations of power unit are endless but
and keep production flowing. These characteristics are also companies like Hagglunds Drives tend to use a modular
important on constant tension systems where the drive has to system with standard components and controls which are well
maintain a constant tension on a wire rope using a winch. matched specifically for drives and can take full responsibility
Hydraulic drives by their very nature have perfect load sharing for the complete drive which adds a security advantage for the
characteristics. If two motors are driving a drum for example, the end user.
pressure from the common pump unit would naturally provide For more information please contact Hagglunds on:
equal pressure to the drive motors effectively sharing the load. tel +44 (0) 1924 220100 or visit: www.hagglunds.com
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
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the drive shaft of the machine. There are no foundation
requirements and no alignment problems. The power unit is
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positioned out of the way with just the hydraulic pipework
connection to take care of. The whole machine look and concept
They can see it online at
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 www.solidsandbulk.co.uk
44 Solids & Bulk Handling • August 2007 www.solidsandbulk.co.uk
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