Motion System Design

Coupling does a big stretch

Flexible power transmission couplings usually accommodate a specific distance between the shaft ends of the machines they connect. But an expandable coupling lets the distance between shaft ends vary by a large amount. In effect, it stretches to connect with a driven machine

For most machines connected by a flexible coupling, the distance between shaft ends is quite specific, for example 1/2 in. on close-coupled drives, 7 in. on pump applications, or possibly 100 in. for a cooling tower fan. In addition to accommodating a certain amount of shaft misalignment, most couplings also allow a specific amount of axial movement between the shafts. This movement can be caused by thermal growth of the shaft or by its tendency to seek to operate at its design center. The amount of movement typically ranges from 0.02 to 0.20 in. for industrial applications. But one high-speed application uses a coupling that accepts a lot more axial movement — over 10 inches!

Test stand challenge

In 1990, a leading manufacturer of automotive test systems asked a coupling manufacturer to design and build an expandable gear coupling for a transmission test stand to be used at a major automobile manufacturer’s plant.

The application requires a coupling (mounted on a motor shaft) that moves horizontally to connect with automotive transmissions on a test stand. Assembled transmissions, traveling along a production line, are positioned close to the test stand, but not always in the same location. Therefore, an axially sliding coupling was needed to compensate for this rough positioning and simplify the testing procedure.

Application requirements include a 7,200 rpm operating speed, dynamic balance to AGMA (American Gear Manufacturers Association) Class 9, and an axial movement capability of 10.5 in. A Class 9 balance permits 0.002 in. maximum radial displacement of the coupling axis during rotation. Therefore, a precision coupling was necessary.

The test system manufacturer subsequently installed 12 of these special geartype couplings. But the fit of the gear teeth and the inherent wearing of a gear coupling due to friction caused unacceptable vibrations in the test equipment. The gear couplings required periodic lubrication, which prevented continuous operation of the test stand. Also, a gear coupling must be taken apart for inspection, causing production delays.

The need for precise alignment between the connected shafts in this application proved to be detrimental to gear coupling operation. A gear coupling needs angular misalignment to enhance the flow of lubricant through the coupling. This is especially true at high speeds where the centrifugal effect causes the lubricant to move away from the gear mesh where it is needed.

Sliding solution

In 1994, the Thomas Coupling Div. of Rexnord Corp., Warren, Pa., offered an alternative: a non-lubricated, sliding coupling with flexible elements. This device is based on the Thomas flexible disc coupling, in which a series of discs is alternately bolted to the hub for the driving equipment shaft, and to a center spacer whose length depends on the distance between the shafts being connected. Then a second set of flexible discs alternately bolts the center spacer to the hub for the driven shaft. The alternating bolting pattern lets the coupling flex under misalignment.

In the sliding coupling, two intermediate hubs replace the center spacer and a polygon-shaped shaft slides axially within the intermediate hubs, Figure 1. Retaining rings in the ends of the polygon shaft limit axial travel to 10.5 in. and keep the coupling hubs engaged with the shaft. In addition, thrust limiting stops at the flexible elements accept axial thrust loads and prevent the flexible disc packs from being stretched or compressed beyond their design limits. The sliding coupling accommodates the same angular and parallel shaft misalignment as a non-sliding coupling while transmitting the required 2,625 lb-in. maximum continuous torque and meeting the dynamic balance requirements for operation at 7,200 rpm.

For test stand use, the coupling hub on the driven side is interference-fitted with an adapter that connects to each transmission, Figure 2. With the test stand motor and coupling turning at about 100 rpm, an operator-actuated mechanism moves the driven coupling hub and adapter horizontally toward the transmission. The polygon shaft lets the coupling stretch horizontally while rotating at the motor speed. Then the adapter engages the end of the transmission, at which time the coupling may be extended as much as 10.5 in., depending on the position of the transmission relative to the test stand. Once each coupling and transmission connection is made, regardless of its axial position, the test stand performs the required torque and speed combinations.

The payoff

Initially, the test stand manufacturer applied four sliding Thomas couplings in August, 1995. These units have been in operation for over one year with no major problems. As the remaining gear couplings wear out, they will also be replaced by the new sliding couplings. In July, 1996, a second test stand manufacturer purchased eight additional sliding couplings for use in new test stands.

One of the main reasons the automotive manufacturer prefers the disc coupling is that it does not require carrying major parts in inventory. In the event of a failure, only the disc pack normally needs to be replaced. By contrast, gear coupling users generally keep a complete coupling as a spare for such critical applications. Also, the flexible elements of the disc coupling can be inspected without disassembly.

To keep the test time short, the user wanted a quick and simple method for the coupling to move axially so it would engage and disengage the transmissions being tested. The change in coupling length needed to occur while the equipment was operating, without requiring shutdown. Therefore, it was unacceptable to have any fasteners, shaft bushings, or shaft clamping devices that require extra, time-consuming operations. The polygon center shaft, sliding within polygon-bored hubs, provided this onthe- move connection capability.

The couplings required close-tolerance machining, grinding, and balancing to obtain the precise fit between polygon hubs and shaft, and the concentricity needed for this application. But they provide cost savings to the user by minimizing test time and eliminating the need for periodic lubrication.

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