Tooth Tips: Stefanie Burns

October 15, 2013

The all-steel hypoid gear technology is recognized for high efficiency, durability, and quiet operation.


In comparison, worm gear drives use softer, bronze gearing, which shortens service life, and they operate at efficiencies that decline as ratio increases. Hypoid-gearing efficiency stays relatively flat and begins to show significant efficiency advantages for ratios greater than 30:1 ratio. For these reasons, companies choosing hypoid gearing technology for their application usually see significant utility cost savings, and a resultant reduction in their carbon foot print.

To calculate the annual utility cost, use the following formula:


 

Durability
The more durable your gearbox, the lower your maintenance and replacement costs. Over a period of several years, a company will also realize total cost of ownership benefits by switching to a gearbox that may initially be more expensive, but has zero maintenance cost (due to its durability) and possibly the option of maintenance-free grease lubrication, available with hypoid gearboxes. Table 1

Lubrication
Oil churning, seal drag, and friction account for most of the efficiency losses in gearboxes. To some extent these three sources are all affected by lubrication. Seals ride on a thin oil lubricant film. Churning losses are due to the gearbox components moving through the oil sump.

In order to increase gearbox efficiency, by changing only lubrication, you must use the thinnest oil that provides adequate film thickness and contains a good Anti-Wear or Extreme Pressure additive package that provides protection when transient conditions do not provide an adequate oil film. Synthetic oils and oils that have an exceptionally low traction coefficient will reduce internal friction losses. In addition, efficiency increases result in lower operating temperatures. The rule of thumb is, for every 20 degrees F decrease in temperature lubricant life doubles. Table 2

According to the US Energy Information Administration (EIA), the United States generated 1,006 billion kWh of electricity in 2007. It is generally accepted that electrical motors account for about seventy percent of industrial electrical power consumption. Assuming that electric motors are all driving gearboxes, every one percent increase in gearbox efficiency saves the equivalent yearly output of an 800 MW power plant. Small changes in efficiency can have a large aggregate impact. Unlike other efficiency-improving ideas, lubrication changes require no changes to existing equipment.

It’s Safe
For applications such as elevators and hoists, engineers will typically choose a hypoid gear design over worm gear for safety reasons. Since hypoid has more gear contact, it offers more durability than a bevel design. Also, because it uses all-steel gearing, unlike the worm gear that uses a softer bronze gearing, the gear will not wear over time. When you have an instance where a gear wears down over time, a gear tooth can fail at any moment, leading to catastrophic failure. In the case of an all-steel hypoid design, the gear tooth will wear more slowly—and in the worst-case scenario you will receive warning indications that a gearbox is about to fail with rumbling and grinding noises. Backlash is always going to increase for worm gearboxes due to the wear in bronze gearing. In addition, special lubrication must be used for worm gears. There are certain lubricants with additives that will attack yellow gears and make them weak. Table 3

This is not to say that worm gears do not have advantages. Worm gear configurations in which the gear cannot drive the worm are said to be self-locking. For higher ratios, worm gears are self-locking—meaning, even Arnold Schwarzenegger could not grab the output shaft and make it move backwards. It is a safety feature that acts as a brake. But when it comes to people movers, be cautious of using worm gears that offer few warning indications of gearbox failure.

Conclusion
While the greatest efficiency loss is typically associated with the interaction of the gears in mesh, other factors and components also influence the overall efficiency of a system. A gearbox manufacturer is expected to create gearbox designs that minimize efficiency losses within the product through a variety of means. Utilizing high quality gearing with superior surface finish on the gear teeth combined with the incorporation of low-friction seals and bearings all serve to maximize the power efficiency of enclosed gearing product lines.

To the user, the most important factor in selecting a gearbox is whether or not that gearbox size is optimized for their application. Size and weight restrictions may dictate what type of shaft arrangements or manufacturing brand they will use for their application. If a gearbox is unnecessarily oversized—specifically, if the power capacity of the gearbox greatly exceeds the power of the applied motor combined with the application service factor, much of the motor power will be used to overcome the constant losses within the gearbox thereby leaving little additional usable power/torque for the application itself. In short, this would be a situation where the speed reducer is yielding a very low efficiency. Conversely however, a gearbox undersized for an application runs the risk of low life expectancy due to overload conditions despite a seemingly high efficiency.

There are many factors that affect efficiency. Due to the durability and longevity of the hypoid gear design, it is often possible to replace worm gearbox/motor combination with a smaller horsepower input hypoid and actually receive greater torque at the driven shaft. In addition, hypoid gearboxes offer the option of grease lubrication that can eliminate oil leakage. By eliminating oil leaks, your operation will be cleaner and maintenance free. Engineers looking to make their operations greener are looking into grease lubricated gearbox designs for this reason. Deworm your operation and replace it with hypoid gearboxes for efficiency improvements in your overall system.

About The Author

Stefanie Burns

is the international marketing manager of Sumitomo Drive Technologies. She has an industrial marketing background of over 10 years, and is a graduate of Virginia Tech.