Computerized assembly increases worm-gear accuracy

Oct. 19, 2000
When worm-gear speed reducers are assembled, proper alignment of the worm and gear, and the resulting tooth contact pattern, is the most critical assembly variable.

A new computerized assembly procedure for worm-gear speed reducers increases accuracy and eliminates the need for highly skilled assemblers.

When worm-gear speed reducers are assembled, proper alignment of the worm and gear, and the resulting tooth contact pattern, is the most critical assembly variable. It is also the most difficult to control. Emerson Power Transmission engineers have developed and patented a new computerized mechanism to quickly and accurately align the worm and gear.

A typical worm-gear reducer assembly includes the input worm shaft straddle-mounted between ball bearings, and the output wormgear and shaft supported by tapered roller bearings. To produce these assemblies with acceptable worm-to-gear contact patterns, assemblers must position the worm-gear under the worm centerline accurately, and at the same time set output bearing endplay within a close tolerance, usually in the range of 0.002 to 0.003 in. This typically requires a trialand-error process that is neither cost effective nor precise. First, technicians apply bluing to the worm-gear, then assemble the reducer with shims either behind the bearing cups or under the retainers. Next, shim-pack thickness and location are adjusted until the correct combinationof contact pattern and bearing clearance is met. This can be time consuming, because the reducer may need to be assembled and disassembled repeatedly, and final quality depends on the skill of the assembler.

Gearing manufacturers have sought to shortcut this process by locating the theoretical worm-gear centerline relative to one bearing shoulder, and then using statistical average dimensions for bearing cup/cone stack height and housing bearing seat locations. With this technique, the same shim-pack is always used behind the rear-bearing cup, so the assembler need only adjust the outboard shim pack for correct endplay. This approach eliminates the need for bluing, but still requires disassembly of the outboard bearing retainer, and the resulting tooth contact pattern is only average at best.

It is commonly believed that a poor contactpattern will self-correct as the bronze gear wears-in over time, but the reality is that part of the worm-gear assembly's useful life is lost in the process. In addition, high contact stresses that result from a small initial contact area can cause premature scoring of the worm thread, which further accelerates gear wear while shortening life. The resulting high concentration of bronze serves to exacerbate the problem.

EPT's Emerson Gearing Div. resolved these various assembly problems by developing a computerized centering device that accurately measures each assembly component and automatically calculates exact front and rear shim-pack requirements.

First, the computer measures stack height of the output assembly. Next, the computer's master worm contacts the bronze gear and oscillates horizontally to find the exact centerline. The computer measures the reducer housing and output bearing retainer to determine the location of front and rear bearing seats relative to the housing face. A computer data screen shows the number and thickness of shims to place behind the front and rear bearing cups. This centers the wormgear and gives the desired bearing endplay.

The computerized process eliminates the need for bluing, dial indicators, multiple assemblies and disassemblies, and highly skilled assemblers. The result is a substantial saving in time, a perfectly centered tooth contact pattern, and accurate endplay settings.

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