Hypoid vs Worm Gear Efficiencies

Sumitomo Machinery Corporation of America 4200 Holland Boulevard Chesapeake, VA 23323 f.757.485.3355 f.757.485.3193 www.sumitomodrive.com

Hypoid vs Worm Gear Efficiencies – White Paper 1

Hypoid vs. Worm Gear Efficiencies By Stefanie Burns

Sumitomo Drive Technologies

November 1, 2009

Sumitomo Machinery Corporation of America 4200 Holland Boulevard Chesapeake, VA 23323 f.757.485.3355 f.757.485.3193 www.smcyclo.com

Improving Energy Efficiency through Lubrication – White Paper 2

Still using a worm gear and trying to improve the overall efficiency of your system? Purchasing a high efficiency NEMA premium motor may improve efficiency only marginally if you pair it with an inefficient gearbox. The motor is only half the equation—you need to also improve the gearbox efficiency if you want to improve your system’s efficiency. By employing a hypoid gear, engineers have created a right angle gearbox that offers high reduction, high efficiency, and compactness that will increase your operating system. It’s All About the Design Hypoid gearing has been around for a while, and you may be familiar with this technology that is commonly used in rear-wheel drive automobiles. Engineers use hypoid gears for the rear wheel axis on cars for two reasons: (1) it moves the drive shaft down which creates more room in the passenger area and

(2) it is simply more durable. A 1965 Mustang that uses hypoid gears will still operate with the same hypoid gear it was originally assembled with—that’s more than 45 years! Hypoid gears have specially formed teeth on a circular face that are driven by a worm-like drive on non-intersecting axes. They are similar to bevel gears, however, bevel gears intersect at a perpendicular axis and call for a larger size gearbox—which is why most of us default to a worm gearbox. As the image above shows, conventional spiral bevel gearing, has essentially a pure rolling, meshing action that is mechanically very efficient. But the drawback lies in its small total tooth contact area, resulting in a low torque throughput capacity. Reduction ratios greater than a 6:1 are not possible in single-stage spiral bevel gearing. However it may be possible to get higher ratios with the aid of multiple-stage configurations, but this comes with the problems of lower mechanical efficiency, increased backlash, more space consumption and greater weight. Conventional worm gearing, whose meshing position is shown at the bottom of the image, has a considerably higher total tooth contact area. Although it offers a high torque throughput and a high reduction ratio, it fares poorly in terms of mechanical efficiency. This is due to the friction generated by its sliding action. The high heat that is generated can raise the lubrication temperature to unacceptable levels when the box is operating continuously for industrial use. In reality, worm gears do not have a particularly good overload capability; rather, their thermal limitations cause them to be operated at loads below their mechanical limits, therefore being inefficient. (Dudley’s Gear Handbook). Worm gearing is also affected by wear that demands regular adjustments in order to maintain accuracy. In contrast, Hypoid Gears offer a rolling action along with a very small component of sliding action. The sliding action is what wears down the gear component and often leads to tooth breakage. When compared to bevel gearing, the hypoid gear shows a greater tooth contact area, which offers more durability. In addition, hypoid gears tend to offer quieter and smoother operations in comparison to spiral bevel gearing. The efficiency of a hypoid gear is less than a similar set of spiral bevel gearing (90-95% as compared to 99% for many spiral bevel gearing), however much higher than worm gears. Worm gear efficiency is quite dependent on operating speed. Typical worm gear efficiencies run from 50 to 90%, (Dudley’s Gear Handbook).



Energy Efficiency In most industries, power consumption in the manufacturing environment accounts for approximately 1/3 of all energy consumed annually within the United States. In the enclosed gearing industry, questions such as “how efficient is product X” are becoming increasingly common. As no mechanical device is 100% efficient, this numeric value of efficiency will always be less than 1. Several factors influence how efficiency is lost during operation of the gearbox system. A). Friction Two gears in mesh incur losses in efficiency due to the sliding action of one gear tooth against the corresponding gear tooth of the mating gear. This sliding action reduces the overall efficiency of the gearset since usable power is converted to heat. It is not accurate to say that a specific gear type has a definite efficiency associated with it, however some gear types typically operate at lower efficiencies than others. Efficiency, as it relates to enclosed gearing, is simply the ratio of the output power (power transmitted through the gearbox as usable work) to the input power. Here is an example from several worm gear manufacturers catalogs:

Worm A Worm B Worm C Hypoid 10:1 88% 90% 90% 90% 30:1 74% 80% 78% 90% 50:1 64% 71% 71% 90%

100:1 55% 60% 57% 85% 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. As shown in graph above, 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:

Cost = Rate x Connected Horsepower x 0.746 x Hours Run Motor Efficiency x Gearbox Efficiency

Where: Cost = Annual utility cost Rate = Local cost of power per kWh Connected Horsepower = Number of operating drives x HP Hours Run = Hours per day multiplied by number of operating days per year



B). 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, which is available with hypoid gearboxes. OPERATIONAL COSTS Year 0 Year 1 Year 2 Year 3 Year 4 Year 5 WORM

Initial Investment ($995.00) - - - - - Maintenance Costs - $200.00 $200.00 $200.00 $200.00 $200.00

Operating Costs - $1,054.64 $1,054.64 $1,054.64 $1,054.64 $1,054.64 HYPOID

Initial Investment ($1,275.00) - - - - - Maintenance Costs - - - - - -

Operating Costs - $807.73 $807.73 $807.73 $807.73 $807.73 One Box Savings Per Year ($280.00) $466.91 $466.91 $466.91 $466.91 $466.91 One Box Savings Over Five Years $2,054.55 TOTAL POSSIBLE SAVINGS $2,054,545.10

Industrial Example: Customer has 400, ¼ HP drives at 1.9 RPM Output, Ratio 900:1

Rate $/ kWh = 0.051 Connected HP = 100 HP to kW Conversion = 0.746 Hours/ Day = 24 Days / Year = 360 Motor Efficiency = 0.698 Reducer Efficiency: Hypoid = 0.85

Worm-Worm = 0.44

Commercial Example: Customer has 200, ½ HP drives at 7.3 RPM Output, Ratio 240:1

Rate $/kWh = 0.082 Connected HP = 100 HP to kW Conversion = 0.746 Hours / Day = 16 Days / Year = 200 Motor Efficiency: Hypoid = 0.85

Worm-Helical = 0.62 Worm-Worm = 0.58



C. 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 10oC (20o) decrease in temperature lubricant life doubles. According to the US Energy Information Administration, the United States generated 1,006 billion kWh of electricity in 2007. (EIA) 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 hoist, 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. And 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 catastrophically at any moment leading to catastrophic failure. In the case of an all-steel hypoid design, the gear tooth will not wear—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. 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 mechanism device 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 effect 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. Reference: Townsend, Dennis, ed. Dudley’s Gear Handbook. Lewis Research Center, NASA: McGraw-Hill, Inc. 1991. Mark Lee, Johnson. Sumitomo Engineering Department

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Hypoid vs Worm Gear Efficiencies