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Application of the Month

Troubleshooting a Leaky Gearbox
May, 2010 - View all Application Examples

Leaking seals are the bane of gearbox users. Since lubrication is the life blood of enclosed drives, maintaining oil levels is critical to long life. When oil starts leaking, corrective action is necessary, but what action? What’s going wrong?

Actually, the answers can be varied as the gear type used, how it’s mounted, the speed it’s running and the environment it’s in.

We had a user of a competitive worm gear speed reducer give us a call this week. He had his gearbox mounted with the output shaft horizontal and his quill mounted motor pointing vertical, below the reducer. His problem was the input shaft seal was leaking and flooding his motor. He was trying to find out why and what to do about it.

Not knowing the exact internal design of this manufacturer’s gearbox, the best we could do was ask some application questions and suggest some things he could look into.

The speed reducer has a 10:1 ratio and was driven with a relatively standard AC motor. However, it was a variable speed application with the motor being overdriven, often up to 2,500 rpm. The ambient condition was normal factory temperature and there was no corrosive elements acting upon it. So, as you can imagine, troubleshooting was not exactly straight forward.

It was determined that the gearbox housing was quite a bit hotter nearer the motor flange than anywhere else. This was not necessarily unusual as a high percentage of gearbox heat comes from the shaft seal friction. And with the oil level on that side of the internal cavity, it was likely retaining more of the heat. By itself this would add a bit of pressure against the seal. Too much oil or a heavier viscosity would make it even worse. In this case, the gearbox had the proper type and amount.

This led to the question of venting. Many gearboxes need to be vented to reduce internal pressure, although not all do. Worm gear speed reducers often require it because of their relative inefficiency. In this case it was vented properly, so it wasn’t likely pressure was the issue. As an aside, the vent should ideally be placed in the upper most position allowed for it.

Another reason seals fail is a corrosive or abrasive element acts on the shaft surface. It could be rust or grit or fibers or other particulates that accumulate where the seal touches the shaft. Either a groove can be worn into the shaft or the seal material can be worn away. In either case, the proper seal pressure or the shaft is compromised, allowing lubrication to be forced out through the looser connection.

This wasn’t the problem for this customer as the motor was plugged into a Nema adapter, which protected the seal surface from anything on the outside getting in. So, on to the next possible reason.

One of the most common reasons seals leak is not the seal at all. It’s actually a problem with the gearbox bearings. If bearings wear due to torque overload or lubrication problems or mis-alignment or excessive overhang load, then they can’t hold shafts and gears in place. If a shaft is deflected from the gear separation forces, it pushes against one side of the seal reducing seal pressure on the other side.

In this regard, we asked the client to check the perpendicular tolerances of the motor flange plate and motor shaft. We’ve occasionally seen this tolerance to be excessive. So, when the motor is inserted into the reducer and bolted down, it exerts side forces on the input bearings, which wear over time and deflect the input shaft.

We never did get a call back from the customer letting us know the outcome of this investigation. It’s likely one of these reasons came to light and was then taken care of. When trouble-shooting, it would be nice if the symptoms directed us to a straightforward solution. Unfortunately, the same resulting issue can emanate from different sources or for different reasons, requiring a step-by-step process to discover the real problem.

Typically, we like to work with our customers in the design phase so that we can review the planned gearbox integration. We often catch a potential problem early in the process. It’s usually something the designer didn’t consider, most often because he wasn’t aware of the importance of the issue.

That is the difference in buying out of a catalog and working with someone like us. DieQua strives to be an extension of your design team. We can help you make the right decisions when they need to be made.

When designing a new machine with gearboxes, gearmotors, speed reducers or gearheads, give us a call. We’ll make sure you get the right gearbox or connecting component technology and offer suggestions so it’s integrated into your design in the right way.

Chris Popp
Director of Marketing

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