Emerson Power Transmission
Worm-gear speed reducers have offered a rugged, adaptable, and cost-effective method of power transmission for nearly a century. But their acceptance in recent years has been somewhat diminished by inherent inefficiency and a reputation for eventually developing oil leaks. Extensive research into these two related issues has pinpointed several contributing factors:
- Power lost due to worm gearing inefficiency converts into heat. This results in relatively high operating temperatures when compared with moreefficient speed reducers.
- Higher operating temperatures can gradually “cure” and harden seal lips, making them lose the flexibility necessary to seal effectively.
- Without adequate venting, high temperatures increase internal pressure which can force lubricant past seal lips or increase lip contact pressure, accelerating seal wear and grooving on the seal journals.
- Breathers or vent plugs eliminate internal pressure buildup, but often provide a leakage path for lubricant. Bubbles of oil form over the air passageway and eventually percolate to the outside.
- The shaft surface under the oilseal lip is critical to effective sealing. Any lead remaining from the turning process acts like an oil pump and causes leaks. While plunge grinding is the most common method for finishing seal journals, the process does not guarantee acceptable surfaces.
- Dressing grinding wheels leaves a microscopic surface “thread” which, under the right conditions, can transfer to the seal journal and create a leak path.
- A rough shaft surface accelerates seal wear. But a finished surface that is too smooth will not support hydrodynamic lubrication of the seal lip. Most oil-seal manufacturers recommend shaft surface roughnesses between 10 and 20-m in. Ra.
To address these sealing and efficiency issues, Emerson Power Transmission (EPT) developed several design and manufacturing recommendations. The resulting gearboxes tend to run cooler and leak free, compared with conventional wormgear units.
The operating efficiency of worm-gear reducers ranges from 50% to as high as 95%. A number of factors influence efficiency, including ratio, input speed, tooth geometry, and lubrication. By far, the most important of these is ratio. A lower-ratio unit (5:1 for example) has more threads on the worm and a higher helix angle compared to a high-ratio unit. Higher helix angles mean less sliding friction and hence, higher efficiency. By comparison, helical gears typically have a 98% efficiency per gear mesh. For example, a double-reduction helical reducer has an efficiency approximately 0.98
3 0.98 = 96%.
Gear-reducer inefficiency converts power to heat. AGMA ( American Gear Manufacturers Assn.) guidelines for worm-gear, helical, and other types of reducers limit the maximum allowable operating temperature to 100°F above ambient, not to exceed 200°F. To stay within these guidelines, worm-gear reducers must be considerably larger than equivalent-rated helical reducers (to dissipate more heat) or rely on auxiliary cooling devices.
Aside from gear ratio, the most important factor that determines efficiency is the contact pattern between worm and mating gears. Manufacturing the reducer housing, and worm and gear subassemblies creates a stack up of tolerances, so the position of the gear-tooth centerline cannot be accurately predicted. Therefore, assemblers must manually measure and adjust the contact pattern by varying the position of shims behind the front and rear bearings on the output gear shaft. This is timeconsuming and costly, so most manufacturers use statistical analysis to predict shim quantity and location. This provides a reasonably centered contact pattern most of the time.
To eliminate contact-pattern inconsistencies, EPT developed an automated centering machine that accurately measures each worm and gear subassembly, as well as the reducer housing and bearing covers. A computer records the data and calculates the thickness and location of shims required to exactly center the gear under the worm shaft and, at the same time, provides the appropriate endplay for the output tapered-roller bearings. This maximizes operating efficiency and minimizes temperature rise.
The centering machine is part of the worm-gear assembly line. An operator loads the reducer housing and bearing cover, and the machine determines the spacing between output bearing seats.
The output subassembly, which includes the worm gear and bearing cups and cones, loads onto a separate station for two simultaneous measurements. The machine applies a predetermined load to opposing bearing cups and measures the total stack height. At the same time, a master worm engages and oscillates the output worm gear to find the exact center of curvature for the gear teeth. The machine then measures the distance between this centerline and the rear-bearing cup.
The computer then compares the output subassembly stack height to the distance between the housing bearing seats and calculates the shim pack required for the desired bearing endplay. Using the worm-gear centerline measurement, it shows assemblers how to distribute shims between the front and rear bearings to precisely center the gear in the housing. Measurement accuracy is ±0.001 in. Total cycle time, excluding loading and unloading, is less than 20 sec.
Although there are dozens of stock breathers and vent plugs on the market, EPT testing found none to be totally effective under all operating conditions. Variables such as input/output speed, direction of rotation, oil level and viscosity, and reducer mounting position all affect breather performance.
In one way or another, all breathers let air flow between the inside and outside of the gear reducer as the unit warms during startup and cools after shutdown. If the breather is completely shielded from lubricant splash inside the reducer, almost any design will be effective. Unfortunately, this is usually not the case, particularly since most gearreducer housings can mount in a variety of positions. If the breather is exposed to oil splash, a bubble of oil typically forms across the breather's inside orifice, and escaping warm air carries it to the outside. Over time, these small droplets accumulate until the unit is visibly “leaking.”
A novel design that uses a simple coil spring mounted in the breather's air passageway solves the problem. The interior of the spring does not provide a continuous surface where bubbles can form, and this eliminates the percolating effect and resulting oil transfer.
SHAFT SURFACE QUALITY
To function effectively and provide satisfactory life, oil seals must have a thin oil film between the seal lip and mating shaft journal. This condition, known as hydrodynamic lubrication, needs shaft surfaces that are not perfectly smooth but have microscopic pockets to help maintain the lubricant film. Although plunge grinding is most widely used to finish seal journals, the resulting surface is not ideal from this standpoint. Other finishing methods, such as shot peening and liquid honing, provide a matte-type finish which is generally more effective than a plungeground surface. However, even these methods often leave raised, sharp corners that separate the resulting microscopic indentations, again providing potential starting points for leaks.
A seal-journal surface that features an inverted shot-peened texture, with raised (rather than indented) spherical lobes and corresponding valleys in between, better supports hydrodynamic lubrication. EPT developed a process that compresses seal journals between burnishing dies that have been peened to create random spherical depressions. This leaves an “orange peel” texture on the journal surface, typically with a 20 to 40-m in. surface roughness.
Under magnification, the surface has closely spaced spherical lobes with valleys between. The spherical lobes provide an extremely smooth surface for the seal lip to ride against while the valleys retain lubricant and promote hydrodynamic lubrication. Tests show this surface increases seal life by four to five times when compared to plungeground shafts.