INTRODUCTION OF SHIMS (II)
Shimming
When shimming, be sure to keep in mind the following best practices and risks to reduce the likelihood of equipment failure.
- Use no more than four shims.
- Never shim more than an overall elevation of .150" in one application.
- Sandwich thinner shims in between thicker shims to protect them
Shim Design Considerations
While shim rings and washershave the same appearance and some shims are even called washers, the two have very different functions — washers are designed to take and spread loads, while shims fill space.
Shims vs. Washers
While both are used to protect machined components, reduce vibration, or act as a sound buffer, washers spread the load of a bolt or screw, and shims take up space and align parallel and angled surfaces of interfacing elements.
Left: Washers are stamped, leaving a rollover edge and breakout edge. Washers cost less than shims.
Right: Shims have smooth corners, flat edges, and parallel faces with flat, consistent thickness
For best results, wall thickness for shims should be at least three times the material thickness.
Preventing Wear
If the hardness of the shim and component faces do not match, the softer face will be subjected to excessive wear. For best results, and to save time and money on rebuilds, shims should be designed into applications to prevent contact between expensive components.
Measuring Space
A set of feeler gauges (or leaf gauge) can be very handy to determine the amount of space that needs to be shimmed.
Risks
When a shim is used a small space is created above and below it. These spaces are compressed when a bolt is tightened or during the operation of a mechanical component. As shims are added, more compression takes place, increasing the risk of slippage or undesirable vibration of a mechanical component.
In addition to unwanted movement, additional spaces also increase opportunities of dirt and corrosion to develop. This can accelerate shim wear and cause premature failure.
Standard Shim Sizes
Shim sizing is often proportional to the size of the anchor bolts. For motor frame applications, shim size can be estimated based on the horsepower of the machine. Some standard shim sizes can also be determined based on standard motor frame numbers.