Design Approach and Testing Methodology for Ball-Type Non-Reverse Ratchet Mechanism for Vertical Motors

Vertical motors are normally used for pump applications. In general, the pump is located in the well casing and is driven with a long shaft that is coupled to the motor shaft by one of the various available couplings, i.e., bolted, self-release, or non-reverse couplings. Of these couplings, in cases where reverse rotating protection and upthrust handling ability are required, a non-reverse coupling is offered. A non-reverse coupling typically consists of Non-Reverse Ratchet (NRR) which is mounted on either the upper half coupling or the lower half coupling of the motor. An NRR mechanism protects the pump and pump motor from damage caused by a falling water column driving the pump in reverse direction after the motor is shut down. It also prevents the motor from driving the pump backwards which could cause the pump shaft to unscrew at the joints. The NRR mechanism allows the rotor to rotate in the correct or desired direction and locks the rotor rotation when it reverses. An NRR mechanism consists of ratchet plate with teeth which is fixed to the top end shield. A carrier which is mounted on the motor coupling rotates along with the rotor. The carrier consists of locking members, either pins or balls that lock or engages against the ratchet teeth. The locking members move or slide in the holes/grooves present in the carrier. In the correct or desired direction of rotation and at lower speeds, the locking members slide or roll over the ratchet teeth. As the speed increases the locking members move into the carrier grooves/holes and allow the free spinning of the rotor. If the motor rotates in reverse direction and the speed drops below the engagement/drop speed, one of the members engages with the ratchet tooth and prevents the reverse rotation. Based on the locking member used, NRR mechanisms are available as pin-type NRR or ball-type NRR. Higher wear and tear, higher engagement speed and sparking are some of the drawbacks with pin-type NRR. The drawbacks of pin-type NRR make ball-type NRR a popular choice. The rapid engagement of locking members develops a huge amount of impact forces and shock. These impact forces and shock can damage and wear the ratchet, carrier, mounting hardware, etc. As the number of engagement/cycles increases, the wear also increases. The calculations of impact forces are very difficult which makes the design of NRR challenging. This paper outlays the design approach, calculation of engagement/drop speed, contact stress, estimation of impact forces using finite-element analysis, and testing methodology for ball-type NRR mechanism.