Rigid Couplings -- Stronger Than Ever

For direct transfer of torque between two shafts,
few products can beat a rigid coupling

by Arthur Stafford, President, Stafford Manufacturing, Woburn, MA

There are several ways to transfer rotary motion between points in a mechanical system without the expense of gearboxes. Flex shafts and universal joints permit motion to continue around corners or into devices that are themselves moving through a narrow angle of action around the axis of the primary rotating shaft. Flexible couplings have received a great deal of attention over the last several years, transmitting torque at a variety of speeds, with and without backlash, while accommodating various degrees of misalignment, both angular and parallel. At the same time, however, a basic rigid coupling continues to be the ideal component for many applications and is undergoing significant enhancements as well.

Direct connection

Rigid couplings are intended to combine two shafts as though they were one. Although they are backlash-free, rigid couplings allow for no misalignment, either angular or parallel. They are ideal for use on unsupported and overhung shafts.

Two-piece rigid couplings transfer torque directly

In the past, rigid couplings came in two basic styles -- machined set-screw types for small shafts, and ribbed, two-piece cast units, for large shafts and high horsepowers. The ribbed style, relatively unchanged in form and function, remains the coupling of choice for large-shaft applications; however, economics argues against its use for shaft diameters of two inches or less. Most innovations have taken place in couplings designed for shafts of up to two inches in diameter.

The machined set-screw coupling, essentially a cylinder with a keyway and set screws, was fine in theory but had a few drawbacks:

• It tended to loosen under vibration.
• The set screws left dimples when tightened into the shaft or keyway, making removal or adjustment difficult.
• Reliance on the keyway for the transmission of torque made it impossible to change the phase relationship of the two shafts.
• Performance was poor on hardened shafts, where set screws could not lock in place.

Recent years have seen the introduction of a whole new class of rigid coupling products that perform efficiently in a broad array of applications. Produced in a variety of materials and configurations, these versatile components are readily available, easy to use and cost-effective. Common to all of them is the use of clamping, rather than set, screws.

A better grip

Introduced as a premium alternative to the set-screw style, clamp-type couplings functioned well on hardened shafts, were easy to assemble or adjust, and performed effectively under vibration and with reversing loads, but without marring shafts.

In its simplest form, a clamp-type coupling is a cylinder, bored for a snug slip-fit over close tolerance shafting. Slots and clamp-screw locations are machined in such a way as to create individual clamping areas for each shaft. The actual clamping power is typically provided by socket-head cap screws which draw the clamp section tightly around the shaft. This results in a hold between coupling and shaft that is a function of the axial force provided by the clamp screws, as well as the coefficient of friction between the shaft and the inner surface of the coupling. This frictional hold results in the coupling torque capacity, which is itself a function of shaft diameter. The axial force provided by the clamping screws is a function of the quantity of screws, the lubricity of the screw threads and the tightening torque applied to the screws. The coefficient of friction between shaft and coupling is a function of material, surface finish, hardness and lubricity of each surface.

As can be inferred from the above, a strict analytical approach to calculating coupling torque capacity would be subject to wide variability and thus be of little use. As a practical guide, most manufacturers offer charts showing typical test results for their products.

The following is a table published by Stafford Manufacturing, Woburn, MA. Screw torque refers to the amount of torque applied to the clamp screws. Coupling torque capacity is defined as the point at which slippage occurs between the shaft and the coupling. These tests were run with steel couplings on standard steel shafting with any excess oil removed by wiping with a rag.

Torsional Holding Power -- Steel Couplings without Keyways

Rigid couplings are ideally intended to provide zero run-out between the two shafts. In practice, run-out is minimized by following a few precautions:

• Shafts must smooth, with no nicks or dings.
• Shafts must be a snug slip-fit into the coupling.
• Clamp screws are tightened gradually and sequentially, allowing shafts to seat.

Since rigid couplings are not designed to accommodate misalignment, they are not recommended for use on mechanically fixed shafts (e.g., shafts fixed by pillow blocks). They are ideal, however, for overhung and extended loads such as pumps, mixer paddles or long, unsupported shafts.

Driven by customer needs, innovation in rigid couplings continues. The future should bring new products that are stronger, lighter, better, and more economical -- but still, by design, remaining inflexible.

For more information:

Circle 430 - Stafford Manufacturing. or connect directly to their website via the
Online Reader Service Program at http://www.1rs.com/012df-430

Enhancements and New Products in Rigid Coupling * Stepped Bores -- In addition to the original straight-through design, couplings are now produced with stepped bores, allowing the coupling of shafts with different diameters, or a mix of inch and metric shafts, a problem-solver when dealing with mixed components.

* Keyways -- Many clamp-type couplings with keyways are sized to accept standard square keys seated in the shafts. These couplings provide still higher torque capacity (as determined by the shaft diameter and key), as well as the ability to fix the angular relationship between the two shafts.

* 2-Piece Couplings -- Similar in function to the 1-piece style, 2-piece couplings are fully split, allowing for assembly without the disassembly of either of the coupled shaft systems.

* 3-Piece Couplings -- Similar to the 2-piece, 3-piece couplings allow one shaft to be removed or adjusted without disturbing the connection between the other shaft and the coupling.

* Heavy-Duty Couplings -- Combining larger size with an increased number of larger screws, these couplings provide up to three times the torque capacity as standard couplings -- without having to rely on keys and keyways. They are popular in more demanding applications frequent reversal, impact loads and vibration.

* Shaft Adapters -- Members of the coupling family, these items clamp on a shaft end and provide a male shaft output. The output side is machineable, allowing the user to extend a shaft or change the configuration of its output end. For example, an inch shaft can become metric, a straight can become tapered, or a smooth can become threaded. These products are especially useful in machine repair and retrofit.

* Phase Adjusting Couplings -- A very special type of rigid coupling, a phase-adjusting coupling allows for very fine and accurate adjustment of the angular phase relationship between two coupled shafts.