Thrust Actuator Designed to Provide Two Degrees of Freedom

This full article was published in the June 2003 issue DPN: Design Product News.
Written by Steven Marinelli, PE - Senior Design Engineer and David Casper, Sales Applications Engineer.

The principle behind hydrodynamic bearings is simple: the rotation of a collar or journal drags oil into an oil wedge creating pressure that supports a load. Albert Kingsbury revolutionized the power generation and propulsion industries almost 100 years ago using this fundamental theory to develop the tilt pad thrust bearing. Since then, it has inspired the advanced bearings produced today.

Kingsbury, Inc., a manufacturer of hydrodynamic bearings, was asked last year to add a new wrinkle to the principle. A customer wanted the company to design and manufacture a tilting pad thrust bearing linear actuation system that would be capable of moving a rotating shaft axially while transmitting a thrust load.

The wrinkle - moving a rotating shaft while transmitting a thrust load - would be a new innovation in the hydrodynamic bearings industry.

This two degrees of freedom system would absorb a 24,000 pound thrust load while the shaft translated axially 3" and rotated at 3,000 rpm. The thrust actuator would do all this while maintaining a linear position accuracy of +/- 0.001".

There were several challenges to overcome in order to meet the customer's design specifications:

Tilting pad thrust bearing linear actuator system is capable of moving a rotating shaft axially while transmitting a load.

1. Determine a mechanism to axially move the bearing while absorbing a high thrust load;

2. Incorporate a method of control with positional feedback to maintain position within the specified tolerance;

3. Design a housing for the lubrication system that provided proper lubrication of the thrust faces, kept the oil contained, and extended no more than 30".

Axially movement was resolved by using a dual acting hydraulic cylinder. This cylinder moves by way of an independent lube oil system. A positive displacement pump continually supplies oil from a reservoir to a proportional valve. Based on the desired position, the valve adjusts the flow to either end of the cylinder. If no movement is required the flow is recirculated through a heat exchanger back to the reservoir.

To maintain position, an active control system was incorporated into the unit. The operator provides a 4-20mA input signal reflecting the desired position, or set point. The input signal is compared to the output from a linear proximity probe. The active control system compensates for the difference in the two signals by adjusting the position of the hydraulic cylinder. This process continues until the linear probe and the set point correspond to the desired position. The cylinder provides the position as well as the necessary force.

Figure 2 & 3 - Special glide rings allow the inner housing to slide relative to the outer housing with minimal friction.

The bearing housing would be open at one end and could not be easily sealed. This challenge of maintaining oil lubrication without an enclosed oil cavity was solved by using the Kingsbury Leading Edge Groove® (LEG) design direct lubricated bearing (Figure 2 - inset). Oil enters the annulus at the back of the bearing base ring and distributes oil to axial holes through the base ring outer wall into the oil feed tube. The feed tube directs oil to the leading edge groove of the pad where it enters the oil film at the leading edge of the thrust pad between the bearing and rotating collar. The discharge oil flows into the customer's housing where it is returned to the main oil system. Since the inner housing is not pressurized, the risk of oil leakage is minimal.

In order to allow for axial movement and keep this assembly compact (less than 30"), the design consisted of inner and outer bearing housings. The inner housing, fitted with special glide rings, allowed the inner housing to slide relative to the outer housing with minimal friction (Figure 3). A specially coated steel key fastened to the outer housing was used to prevent the inner housing from rotating and kept the part aligned. The key slot machined in the inner housing allowed the unit to slide.

In addition to meeting the challenges presented by the customer's specifications, Kingsbury designed the thrust actuator using commercially available products. This helped reduce manufacturing costs and speed delivery. The unit was installed late last year and ran flawlessly.

Figure 2 & 3 - Special glide rings allow the inner housing to slide relative to the outer housing with minimal friction.