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Gearbox Lubrication

Steve Koen
10th June 2009

Background

Bath Lubrication

As a general rule, gearboxes are often lubricated by a simple combination of bath and / or splash. Bath lubrication is the simplest method, where any component requiring the presence of oil (the expected lubricant in these cases) will be either partially or completely submerged in the sump oil. Bath lubrication is usually used in medium sized gearboxes where all shafts are on one horizontal line and the reducer remains horizontal. Simple and inexpensive, but prone to failure if oil levels drop.

Splash Lubrication

Splash lubrication is used where the gears inside the housing are on various levels, rendering the bath method as useless. Splash lubrication is common in most geared motor style speed reducers due to their relatively small free area inside the housing relative to the oil volume. Again, simple and inexpensive and usually effective in smaller gearboxes with higher peripheral speeds on the gearing to generate the oil mist.

Output Shaft, Bearings

Some helical bevel geared motors (Flender, SEW, Hansen and Bonfiglioli) have the output shaft and bearings on a different horizontal centre line to the other gearing. In fact, their bevel sets are completely submerged in the lubricant which at input speeds higher than 4 pole can cause oil foaming. In these cases, the output shaft bearings have the minimal lubrication possible, i.e. they are grease packed during initial assembly and not lubricated again until dismantling is required. These greased output shaft bearings are often the first point of failure in these drives.

Speed reducer applications - lubrication

A large portion of speed reducer applications require more than simplistic bath and / or splash lubrication. Drives on a luffing boom will change their centerline from horizontal, thus requiring a forced lubrication system. Some drives will have gearing which has a high rim speed, effectively flinging any bath or splash lubrication from the gear mesh mating surfaces. Some manufacturers have ranges of medium to large speed reducers where the horizontal length is kept to a minimum, at the expense of the height. As an example of this, Hansen has their HP1, HP2 and HPP ranges of Compact reducers where the housing centerline split is vertical. These 'bathtub' designs have the gearing on different horizontal levels, which means that an oil bath system will not be adequate.

The problem

In cases like these Compact versions, an alternative needs to be found to ensure sufficient lubrication gets to all bearings, especially the intermediate shaft bearings which are not only physically located higher than the usual oil levels, but still rotating at a relatively high speed. Splash lubrication will rarely feed sufficient oil to these bearings, and grease lubrication has it's own shortcomings in that excess grease will contaminate the oil remaining in the sump. Contaminated oil will decrease the life expectancy of all remaining bearings, and will reduce the load capacity of the gearing. The gearbox will have a relatively short service life.

Alternative solutions

One way around this is to feed oil to these otherwise lubricant starved bearings. While we are at it, we should mention that the pair of face to face taper roller bearings used on the bevel input shaft, as well as the double row spherical roller bearing taking the gear separation forces on the same shaft, will also suffer if sufficient oil lubricant cannot get to them. Oil is needed not only for it's lubricating qualities, but also to cool the (comparatively) high speed bearings and also flush the bearings continuously to minimise contamination.

Mechanical Pump

So we can benefit from a form of pressure lubrication, in fact the terminology is incorrect in that we don't need pressure but we do need positive lubrication - an oil pump is required. The simplest solution is to fit a mechanical pump to one of the intermediate shafts, so that if the shaft rotates, oil is being pumped. One detrimental area is that for the first few rotations, lubricant does not necessarily get to these bearings, until the mechanical pump has pushed sufficient lube in to fill the lines. We get around this by using an electric motor driven pump, where we set a starting sequence such that oil must be available in the lines to allow the drive to commence operation.

Other Options

If we go to the extra trouble of fitting an oil pump, irrespective of the method of drive, we then have other important options available to us. Since contaminants enter a gearbox by the breather, the dipstick and sometimes the seals, we could fit an oil filtering system as well. Maybe a double filter system, so that if one becomes blocked, we can either feed the lube back via a bypass line or through another filter.

On some drives, the heat generated by the gearing inefficiencies can be substantial. So we also need an oil cooler system, complete with an air / oil radiator or perhaps a water oil transfer system. Each has benefits and down sides, so, a case by case study is required to determine which is the better solution.

Conclusion

The aim of improving the lubrication system is to prolong the service life of a speed reducer. How much effort is expended in designing and supplying the system is a direct function of the benefits gained in longer service life. DCL Engineering Group would be pleased to discuss these matters further with you at your next opportunity.

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