How are hydraulics used with bearings?

Bearings are precision components that need to be handled with care during installation or removal (mounting and dismounting). The method of using hydraulics with bearings for installation and removal is applied only when provided for in the design of the mounting. There are two main processes that can be used:

– Apply force on the ring that needs to be moved
– Create a “cushion” or film to overcome the friction force to move the bearing

Applying the Force to the Inner Ring
The tool used to apply the force to the bearing is called a hydraulic nut. It is essentially an annular piston, typically with threads on its bore that allow it to be fixed to the shaft or sleeve to anchor it when applying pressure to the bearing inner ring. Hydraulic nuts are only used when the application requires an interference fit with the inner ring (including driving the bearing up a taper, which in truth creates the interference fit).

Some hydraulic nuts have no threads on the bore. In this case, the nut is anchored to the shaft by using an end plate on the shaft for the body of the hydraulic nut to push against.

Conceivably, a specially designed hydraulic nut could be made to push the outer ring of a bearing into its tight-fitted housing. Since a tight-fitted outer ring is relatively uncommon, there are no standard hydraulic nuts designed for this. However, the principle remains the same: the body of the hydraulic nut would be fixed to the housing and the piston would apply force to the outer ring.

Creating a Cushion
A hydraulic oil film applied between the bearing and its seat reduces the frictional sliding force and allows easy installation or removal of the bearing. In reality, the hydraulic film plays a dual role. It creates an oil film and it stretches (or compresses) the ring. In both cases, the amount of force required to install or remove the bearing is reduced.

To apply this force, a channel needs to be drilled through the shaft (or housing) that intersects with a radial channel that feeds a circumferential groove on the bearing seat. This must be done when the shaft or housing is first manufactured or reworked. It cannot be done as an afterthought or at the time of mounting or dismounting.

When it comes time to install or dismount, the hydraulic pressure is applied between the ring and seat, and then the bearing is pushed or pulled on or off the shaft/housing using some other method of force like a press or puller.

Combination
Some assemblies will have both features: a hydraulic nut to apply the installation pressure and a hydraulic shaft or housing feature to reduce the force needed for installation.

What’s the Benefit?
By using hydraulics to install or remove bearings, there are significant savings in several areas — time taken, safety, and avoiding damage to the bearing or surrounding components. To put it simply:

During installation, you avoid bashing a locknut with a hammer and drift.

During removal, you avoid using a torch or zip cutter, which eliminates fire risks and the need for additional work permits.

Why Isn’t This Standard on All Bearings?
The reason not all bearing applications come predesigned with hydraulic installation and removal features is due to cost allocation. Shafts and housings with these hydraulic design features would need to come from the original equipment manufacturer (OEM). These are capital expenditures, but the cost savings occur later in the maintenance and operations budgets, which are not always considered during initial equipment specification or purchase decisions.

Where Are Hydraulic Features Common?
Some industries make hydraulic features standard — most notably the paper industry. This is because the bearings used on paper machine rolls are large, expensive, and regularly changed (for roll re-surfacing operations). The ability to efficiently install and remove bearings is critical to maintaining efficient operations.