Forced lubrication systems keep things moving

There are several reasons for installing a pumped oil system on any piece of mechanical equipment, lubrication and cooling being the most common, reports Richard Tipping of Tuthill Pump Group.

The increasing amount of input power being used especially in gearbox and transmission equipment is leading to a requirement for forced lubrication systems.

Also, such systems are now being used in conjunction with oil cooler to provide temperature control over the lubricants, as an essential factor in extending the life of the lubricant and consequently of the equipment it is being used on.

When engineering a pump into an application the lubricant is a major factor for consideration.

This is because there is a correlation between viscosity and pump speed that impacts on pressure loss and inlet velocity on the suction side of the pump and for efficient performance this should never exceed 2m/s, even on low viscosity fluids.

Consequently, it is important that the pump manufacturer is advised of the lubricant selection by the designer of the equipment's the lubrication system.

We are now seeing the dominance of synthetic and semisynthetic lubricants; these are steadily replacing the more traditional straight mineral oils especially in the compressor and gearbox industries.

These lubricants have an advantage over traditional mineral oils in terms of an extended working life and a higher carbonisation point and so can be used safely when the equipment is working at higher temperatures.

Despite the performance enhancements offered by synthetics, they have similar physical characteristics to mineral oils in terms of viscosity etc so far as pump performance is concerned.

However, the engineer must ensure that the correct elastomers are used on all sealing surfaces.

This is because some synthetic oils, unlike mineral oils, can have a detrimental effect on elastomers that will cause degradation leading to seal failure.

In the majority of cases a positive displacement pump such as the internal gear design from Tuthill will be used.

A gear pump is particularly well suited to this type of application as it has the ability to prime itself with fluid so enabling the designer to mount the pump above the oil level.

In addition a gear pump is capable of handling oil when it is at the extremes of its temperature range - cold and viscous or hot and thin.

Added to these benefits is the ability to easily fit the unique Tuthill reversing feature on the internal gear design, making it an excellent option for this application.

Another consideration when using a positive displacement pump is its ability to develop pressure, too much pressure can be damaging so a relief valve needs to be incorporated either within the pump or within the lubrication circuit.

Many pumps have the option for fitting a valve but in certain cases where the pump is very small this is not physically possible so the protection must be built in by the system designer.

Care must be taken when using relief valves on systems using highly viscous oils to ensure the valve is sized correctly unless the potential back pressures generated within the valve will render it ineffective When the decision has been made to install a forced lubrication system the choice is then made as to which type of unit is to be used.

If the requirement is for the components to be lubricated before the equipment starts to rotate, such as in a turbine, then a motorised oil pump will be used.

This is to ensure that there is sufficient pressure within the system before start up, which is generally achieved by the fitting of a pressure switch that locks out the main driver until a set system pressure is achieved If, however, the equipment is running at lower speeds or has been designed to retain oil around critical areas, a direct mounted pump will be used.

Here there are several options open to the designer, the principle one being does he want to install the pump on the outside of the equipment, such as cartridge type-pump, or does he want to integrate the lube pump into the equipment, thereby avoiding exterior pipe work.

Cartridge pumps can either be bolted onto the exterior of the equipment to be lubricated or inserted into it with just the cover exposed.

Such low profile characteristics are important if the equipment is to be installed in hazardous environments where the pump can be vulnerable, such as in quarrying and mining.

Both internal and external porting arrangements are typically available with this type of pump.

Unlike conventional pumps, integrated units comprise a combination of components that are fitted onto and driven by a shaft that is part of the equipment that requires lubrication.

They have no body parts or separate shaft.

This kind of pump is a good solution for OEMs looking for space efficient solutions and where simplicity of design and installation is required.

In some instances, the equipment to be lubricated might be required to operate in both directions but obviously lubrication flow must be maintained in the same direction.

For motor driven pumps this is not a problem but for applications with pumps driven from a shaft on the equipment being lubricated a solution needs to be determined.

There are various techniques for overcoming this situation.

Most demand additional and some times costly combinations of non-return valves to maintain flow irrespective of the rotation of the driving shaft.

However, there are simpler solutions, such as the unique reversing system from Tuthill that requires just an easy internal modification to a standard pump.

It is a purely mechanical device that is powered by the friction within the pump.

This design overcomes the problems that can be associated with valved systems when the fluids become contaminated.

This unique system has over 50 years of proven experience on arduous duties, particularly dusty environments, especially gearboxes employed on off-road and military vehicles, and mining and quarrying equipment.

Finally, having identified the type of pumps most appropriate for achieving the applications lubrication requirements, it might be necessary to factor in the location of the equipment's installation.

In an offshore or petrochemical environment, materials of construction will play a major part in the pump specification.

For although conventionally cast iron is the standard material for the casing of lube pump for such applications these, due of the aggressive and explosive nature of the environments, almost invariably steel will be employed.

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