True Hold ^® Low Speed Backstops: How It Works

Backstop Operation

Overrunning

Figure 1 - For over 99% of its operating life a backstop is in the overrunning mode of operation.

The rollers, roller cage and stop lugs rotate with the inner cam as a unit since they are connected by the energizing springs.

The outer race does not rotate since it is bolted to the end covers, which are held by the backstop torque arm.

While overrunning, the rollers roll on the outer race and slide on the inner cam ramps. Friction and centrifugal force tend to lift the rollers off the cam, minimizing contact and wear (Figure 2).

The energizing springs stretch during overrunning to provide tension to the roller cage assembly (Figure 1). This tension keeps the rollers ready for instantaneous backstopping engagement and minimizes the relative rotation of the roller cage to the inner cam.

The stop lugs axially position the roller cage assembly on the inner cam. They also prevent the roller cage from rotating too far which would cause the rollers to strike the upright side of the adjacent ramp. Maximum relative rotation of the roller cage assembly and inner cam during overrunning is between .040" and .100", depending on size as limited by the stop lugs.

Backstopping

Figure 3 - As the rotating shaft stops and attempts to reverse, the inner cam is instantly stopped by the wedging action of the rollers in the annular openings between the cam ramps and outer race.

From the outer race the backstopping torque is carried through the end covers to the torque arm and the adjoining superstructure.

Figure 4 - All rollers are engaged simultaneously since they are positioned by the spring loaded roller cage.

Load division between the rollers is assured by machining accuracy of the inner cam ramps, rollers, roller cage and outer race.

As additional backstopping torque is applied to the inner cam, the rollers will tend to move deeper into the wedging position, thereby increasing the resistance to slippage.

Relative rotational movement between initial backstopping engagement at no load to backstopping at full catalog rating is approximately 1 ½ to 3 degrees.

The torque capacity of the backstop is based on the tangential friction resistance force at the outer race developed by the compressive force between the inner cam ramps, rollers and outer race.

The maximum torque capacity of the backstop is limited by the Hertzian contact stress at inner cam / roller and roller / outer race contact points, bending strength of torque arm, and hoop stress of outer race.