Preloading is the application of external loads exerted on precision systems and assemblies to ensure a smoother and more controlled motion. Preloading also prevents unwanted motions such as backlash, clunking, buckling, and unwanted axial motion in systems such as axles, transmissions, and power takeoff units (PTU). Preloading shims are placed between mating parts in a variety of automotive, heavy duty truck, and other transmissions, axles, differentials, and PTUs to adjust the space between precision components.

By ensuring proper space, the preload shims are adjusting for tolerance stack up within the supply base of the mating parts and allow buyers and engineers to reduce the overall cost of their purchased parts by increasing their allowable tolerance. This is why it is so critical to have tight tolerance and high quality for any preloading shims, precision shims, adjustable shims, tight tolerance shims, and selective shims.  After the parts are assembled and the “gap” is determined, a selective pre-load shim is installed to make up this gap, ensuring proper torque to turn and backlash ratios.  If the shim experiences too much preloading, it means that the mating parts are too tightly connected together and this will induce additional external loads, drastically reducing fuel efficacy, bearing life and increasing heat generation.

Since adjustable shims need to have variance for their thickness for tolerance stack up, they won’t require tight tolerance for manufacturing. Other types of shims including precision shims, tight tolerance shims, and selective shims can also be used for preloading. Preloads can be fixed to the preferred amount with just a few selective or adjustment shims, but customers will have many more than one thickness to choose from, since the final stack up varies assembly to assembly and is not known until the mating components have been assembled. Although there is a wide variance of thickness for the shims, there will still need to be a requirement for tight tolerance and high quality for automotive customers to control the spacing between parts.