How to determine joint size

March 6, 1997
Block-and-pin universal joints are relatively hassle-free components, provided they’re properly sized for the job. The simplest sizing involves replacing one U-joint with another of the same type
William G. McCombe
Curtis Universal Joint Co. Inc. S
pringfield, Mass.

Block-and-pin universal joints are relatively hassle-free components, provided they’re properly sized for the job. The simplest sizing involves replacing one U-joint with another of the same type. Here, sizing is merely a matter of measuring the outside diameter of the existing joint. Measuring the shaft — which is generally equal to one-half the diameter — is another option. In either case, the inside diameter shouldn’t exceed 60% of the OD. For new applications it is necessary to consider actual operating criteria. The steps are straightforward. Designers start by determining a required torque and then select a joint that meets or exceeds the required performance level. Here are the steps:

• Multiply shaft speed (rpm) by the angle at which the joint must operate to determine the speed-angle factor.
• Select the corresponding operating use factor from the use factor table. If the result exceeds 15,000, the speed-angle factor is too severe for a block-and-pin universal joint.
• Calculate the input load Lin on the joint:

force pushing lubricants away from the bearings.

One way to avoid such problems is to use molybdenum disulfide-based lubricants, which posses excellent adhesion and retention qualities. For additional protection, an oil-resistant boot placed around the bearings encapsulates the lubricant and keeps it clean for an extended period of time.

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