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SHIMS CAN PROVIDE COST- AND LABOR-REDUCTION BENEFITS DURING ASSEMBLY, INSTALLATION, REFURBISHMENT AND RETROFITTING
Shim is a generic name for a variety of components and configurations that range from simple OD/ID shapes (such as flat washers) to complex geometries (Figure 1). Shims are typically created by stamping with a punch press, cut via laser, EDM or water jet, or machined. The type of shim material, the configuration and volume of the part will generally determine the optimal manufacturing method.
Precision shims are commonly used in generators, motors, gearboxes and torque converters, turbines, compressors and pumps, for example, in the oil and gas industry. They can provide cost- and labor-reduction benefits during assembly, installation refurbishment and retrofitting. A few ways shims can be used:
• Space Compensators: Accumulated tolerances can cause axial motion in a shafted assembly (motors, generators, pumps, gearboxes or torque converters, for example), which can reduce assembly life. Precision shims can be used to absorb these tolerances
• Thrust Washers: Shims can act as thrust washers, an economical alternative to roller thrust bearings when applied forces are not extreme
• Surface Mating: Precision shims can be installed between the mating surfaces of two components or assemblies. Compared to directly mating the assemblies, they offer lower manufacturing costs as mating surfaces on castings or machined components can be manufactured to less precise tolerances; shims are used to compensate for any space created
• Sacrificial Shims: When a shim for mating surfaces is made of a slightly softer material than the mating assemblies, it can act as a sacrificial plate by absorbing the wear that would normally affect the mating surfaces due to normal friction, wear and tear. This method reduces turnaround time during equipment rebuild or retrofit operations
• Leveling Shims: Sometimes called pump shims or foot shims, leveling shims are installed at the base of a mating component to assure the perfect alignment of two assemblies. This prevents angular misalignment between rotating components which could cause premature failure.
Shims can be made from solid or laminated material. Solid shims are best when the thickness of the shim is the same in every assembly. Surface bonded laminated shims are made from multiple layers of precision gauge metal foil, with the surface of each layer coated with a resin bonding material.
Laminated shims are the optimal solution when the required thickness falls somewhere within a predictable range. Laminated shims offer the advantage of fewer SKUs and less warehouse space compared to stocking a multitude of shims of different thickness. A manufacturer can warehouse a shim of the maximum required thickness, and then peel foil layers to suit the thickness requirement of each assembly (Figure 2).
For example, if the required thickness of a thrust washer always equals .062”, a solid shim would be best. However, if the stacked tolerance fell into the range of .032” to .100”, a laminated shim of .100” thickness would meet the needs of any thickness within the required range. The drawback is that the material that is peeled away must be discarded. Depending on the size, configuration and material of the shim, varying degrees of proficiency may be required to peel shims.
Edge bonded laminated shims have all the advantages of surface bonded shims, without the drawbacks. Instead of being bonded at the surface of the foil, they are only bonded at selected locations on the edges of the shim with a pliable cement product that is easier to peel.
In addition, the portion that is peeled can be reused. This is particularly beneficial in field service applications where the technician is not proficient in the peeling process (Figure 3).
While shims can be used to compensate for an unanticipated tolerance gap, they can save money and lessen inconvenience by being incorporated into the assembly during the design process.
Guy Prentice is the Shim Product Specialist at Spirol West. For more information, visit www.spirolshims.com