Motion System Design

Shaft keys revisited

Dimensions of shaft keys, standardized many years ago, are commonly considered to be the main concern in applying these devices. However, designers tend to forget that other important factors exist

Keys are usually made on the shop floor by mechanics in accordance with published standards. However, this simplified approach is likely to produce keys made of the wrong material and in the wrong shape. The reason: published standards only cover key and keyway dimensions. Most standards fail to discuss the important issue of key material, as well as their shape and installation.

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Key material

To avoid key failures due to overload, choose a key material with the same strength and hardness as the shaft or hub. The reason for this can be found by examining the equations for calculating key stresses.

As shown by the last equation in the box, key stress equals shaft stress when the key (or hub) length is 1.6 times the shaft diameter. But modern couplings, particularly those made from alloy steels, have shorter hubs than this. They are usually equal to, or only slightly longer than the shaft diameter. In this case, key stress is about 50% higher than shaft stress. This would appear to require that the key material be 50% stronger than that of the hub; however, because part of the torque is transmitted through friction between the shaft and hub bore, the key material need only be as strong as the shaft material. Key stock is available from most steel suppliers in various grades of steel, including alloy steel.

Key geometry and fit

An improperly fitted key can cause costly maintenance problems, or even machine failures. Before installing a hub for a drive component, ensure that the key has the correct shape and dimensions. Then make sure that the key fits properly as follows:

• Tight in the shaft keyway.
• Sliding (not clearance) fit in the hub keyway.
• Clearance fit radially — a small clearance between top of the key and bottom of the hub keyway.
• Key length extends inward from the shaft end (never beyond) to beyond the hub end, by at least the rounded portion of the key.

The first two of these conditions ensure that the hub can’t rotate, even slightly, on the shaft. If a hub rotates (slips) on its shaft, every torque reversal causes hammering on the sides of the key, leading to damage. As damage to these contact surfaces occurs, clearance develops between them, and the key will eventually shear. A sliding fit between the key and hub keyway is recommended for ease of assembly and disassembly and can cause a hub to split. To ensure that the hub seats properly on its shaft, provide some clearance between the key and the bottom of the hub keyway. In the case of a coupling hub, this clearance provides an opening through which water or corrosive gases can enter the coupling, causing damage to its internal surfaces. To seal this opening against contaminants, apply a room temperature vulcanizing (RTV) sealant on top of the key before installing the hub.

The corners of a key must be chamfered so they do not interfere with the fillet radii in the keyway, Figure 2. On the other hand, too-large a chamfer reduces the area of contact between a key and the sides of the keyway. Under load, and particularly during shock loads, these smaller contact surfaces are more easily damaged.

A loose-fitting key, Figure 3, allows forces generated by torque to roll the key, causing high edge loadings between the key and keyway. Such edge loading can shear the key, Figure 4.

Tapered keys (plain or Gib-head) are sometimes used instead of setscrews to hold hubs from sliding on their shafts. Driven in too far, they cause poor hub-toshaft contact, Figure 1; if not driven far enough, they allow sliding between hub and shaft. Unfortunately, there is no way to check if tapered keys are properly installed. For these reasons, you should avoid using tapered keys for coupling applications.

Design, manufacturing, and installation errors

A key with a square end is likely to damage the shaft when transmitting torque, Figure 5. The corner of the key creates a high contact stress in the shaft, which can cause it to fail. To avoid such failures, use only keys with rounded ends.

The transmission of torque through a key must occur over the full length of the hub, otherwise twisting movements can occur between the hub and shaft. These movements cause shaft fretting (surface damage caused by small alternating motions), which leads to fatigue failure. If a key has a round end, the only way to transmit torque over the full length of the hub is to extend the key beyond the hub, by at least its rounded portion. Figure 6 shows the wrong approach: although the end of the key is rounded (as it should be), the key transmits torque only over its straight portion.

The relationship between key length and hub length also affects system balance. For example, a void between the rounded end of the key and the end of the hub keyway, Figure 6, causes the system to be out of balance. This condition can cause vibration especially at the high speeds typically encountered in electrical motor operation.

On the other hand, a key that extends beyond the hub also causes imbalance. If a balanced coupling is required, use a notched key, Figure 7. As a rule-ofthumb, a coupling should be balanced if it is going to be installed on the shaft of a motor with a balanced rotor.

The most often encountered error in key manufacturing is the use of key steel that is softer than the shaft. Remember that the key material and hardness should be similar to that of the shaft or hub.

This article is based on the book “Flexible Couplings: their design, selection and use,” by Michael Calistrat.

Michael M. Calistrat is a consultant on power transmission design and failure analysis and owner of Michael Calistrat & Associates, Missouri City, Texas.

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