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Avoiding Gearbox Bearing Failure Due To Lack Of Lubrication

By: Chris Popp
Director of Sales and Marketing
DieQua Corporation

There are many reasons gearbox bearings fail. From torque overload to excessive radial load to shaft misalignment, there are a myriad of things to protect against. But one of the reasons that is often lost on many design engineers is lack of bearing lubrication. To avoid this issue the mounting configuration is a critical design consideration.

I suppose I should qualify by saying that in many gearbox designs the mounting configuration is critical. There are some gear technologies, like most planetary servo gearheads and some worm speed reducers, which can be mounted in pretty much any orientation. However in the vast majority of other gearbox designs the mounting position has to be known prior to assembly.

Why is that? It’s because, with regard to lubrication, gravity is not always our friend.

Ok, that’s pretty vague. Lets take a look at an example of a right angle shaft mounted reducer, either as a gearmotor or as a flanged reducer with a separate plug in motor. Nothing looks cooler than having that motor sticking straight up in the air like a ribbed smoke stack. Seems to save more space too.

So, what’s wrong with that? Probably nothing if it was prepared properly. Basic gearbox construction has input gears mounted on input shafts, which are supported by input bearings. If the input shaft is at the top of the gearbox, where is the input bearing? It’s at the top of the box, of course. If the bearing is at the top of the box, where do you think the oil level has to be to keep it lubricated?

To cut to the chase I can advise right now that the best mounting position for almost all gearboxes is with all shafts horizontal. The motor would then be horizontal too. This configuration allows for the least volume of lube to cover all the gears, bearings and seals for maximum splash lubrication and minimum oil shear.

Mounting the motor vertically down also assures a lower volume of oil. The large output shaft and bearings only have to dip into the reservoir to maintain an oil film. The downside of this configuration is that the high speed input gear stage is completely submerged creating addition shear friction. This could cause increased temperature and reduced efficiency.

Another disadvantage is if there is an input seal failure the motor would get a bath from the leaking oil.

Both these configurations are likely preferable to having the motor mounted vertically up, which seems to be the most popular among engineers not in the know. Without other considerations, the entire box needs to be filled to the level of the highest gear or bearing. This requires a lot of oil and leaves little space for pressure expansion.

Also, because of the increased resistance from the large amount of oil churn, efficiencies would tend to be slightly lower. And any loss of oil could drop the level, starving the uppermost rotating components.

One solution is to install sealed bearings in the top positions, which is a common option for some designs. Sealed bearings have a dedicated grease reservoir and don’t draw from the main gearbox sump. Upper most gears still need to maintain an oil film either from splash or misting.

Another possible solution would be an oil circulation system that fed oil in the top and pumped it out the bottom. But that can get pricey and should be considered after other options.

The big take away from this article is that there isn’t necessarily a bad way to mount a gearbox, although some ways are more foolproof than others. It’s that you have to consider the ramifications of lubrication requirements every time you design a gearbox into a piece of machinery and then take proper precautions.

But certainly I can advise not to change your mind how you are going to mount the gearbox on the machine assembly floor after your supplier built the gearbox for a specific or different mounting arrangement. We have received way too many phone calls from customers not understanding the consequences of those decisions, then paying for it way too soon after installation.

08/2014

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