High on Hypoids

Trying to improve your overall efficiency? A high-efficiency NEMA premium motor helps only marginally if it is paired with an inefficient gearbox, such as a worm-gear drive. The motor is just half the equation - the gearbox is also critical. That's why, today, there is heightened interest in hypoid gears. They are key components in right-angle gearboxes with high reduction ratios, high efficiency, and compact size.

Hypoid gearing has been around for years. It is commonly used in rear-wheel-drive automobiles for two reasons: It lowers the drive shaft for more room in the passenger area, and it is more durable than competing designs. For instance, it's not unusual to see a 1965 Mustang still running with the original hypoid gears.

Hypoid gears have specially formed teeth on a circular face that are driven by a worm-like drive on nonintersecting axes. They are similar to bevel gears. However, bevel gears intersect at a perpendicular axis, and that tends to take up more space. So most designs default to a worm gearbox.

Comparing designs

Design engineers have several options for right-angle gearboxes.

Spiral bevel gearing has essentially a pure rolling, meshing action that is mechanically efficient. But the drawback lies in its small total tooth contact area, resulting in low torque throughput capacity. Reduction ratios greater than 6:1 are not possible in single-stage, spiral-bevel gearing. It is sometimes possible to get higher ratios through multiple-stage configurations, but this lowers mechanical efficiency and increases backlash. Multi-stage gear boxes also weigh more and need more space.

Conventional worm gearing has a considerably greater total tooth contact area. Although it offers high torque throughput and a high reduction ratio, worm gearing fares poorly in terms of mechanical efficiency. This is from the friction generated by its sliding action.

Friction in industrial gearboxes that operate continuously generates heat that can make lubrication temperature rise to unacceptable levels. And, according to Dudley's Gear Handbook (Lewis Research Center, NASA: McGraw-Hill, Inc. 1991), worm gears do not have particularly good overload capability. Rather, thermal limitations force them to operate at loads below their mechanical limits, thus making them less efficient. Worm gears also wear over time and that exacerbates backlash. This demands regular adjustments to maintain accuracy.

Hypoid gears offer rolling contact along with slight sliding action. The sliding action does cause light wear. When compared to bevel gearing, however, hypoid gears have greater tooth contact area, which offers more durability. In addition, hypoid gears tend to run smoother and quieter than spiral bevel gears. According to Dudley's, efficiency of a hypoid gear (90 to 95%) is less than a similar set of spiral bevel gearing (up to 99%). Worm-gear efficiency, on the other hand, depends heavily on operating speed and typically runs from 50 to 90%.

Energy efficiency

Some estimates show that manufacturing operations consume about one-third of all the energy used annually within the U.S. Thus, the efficiency of enclosed gearing is getting a lot of scrutiny. Several factors influence gearbox efficiency.

Friction.

Two gears in mesh generate losses and inefficiency from the sliding action of one gear tooth against the corresponding tooth of the mating gear. This sliding action converts usable power to heat and reduces overall efficiency of the gear set. It is not accurate to say that a specific gear type has a definite efficiency, but 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 are examples from several worm-gear manufacturers' catalogs:

All-steel hypoid gears are recognized for high efficiency, durability, and quiet operation. In comparison, worm gear drives use softer, bronze gearing which shortens service life, and they are less efficient as ratio rises. The Comparing gear efficiency graph illustrates that hypoid gear efficiency stays relatively flat and begins to show significant advantages over worm gears for ratios greater than 30:1. For these reasons, hypoid gearing can be less expensive in many applications, with a less severe carbon foot print.

Calculate the annual utility cost C using the following equation:

C = 0.746RC_hH / M_eG_e

where R = Rate or cost of power per kWh; C_h = Connected horsepower, the number of operating drives multiplied by their horsepower; H = Operating hours per day multiplied by number of operating days per year; M_e = Motor efficiency; and G_e = Gearbox efficiency.

Consider a commercial example with 200, half-horsepower drives running at 7.3 rpm output and a ratio of 240:1. Rate = 0.082 $/ kWh; connected hp = 100; hp to kW conversion = 0.746; and the drives run 16 hr/day for 200 days/year. Motor efficiency = 0.698; hypoid reducer efficiency = 0.85; worm-helical efficiency = 0.62; and worm-worm reducer efficiency = 0.58. As shown in the graphic, hypoid gears produce significant cost savings.

In another case, an industrial application has 400, 0.25-hp drives running at 1.9 rpm output, and a ratio of 900:1. Rate = 0.051 $/ kWh; connected hp = 100; hp to kW conversion = 0.746; and the drives run 24 hr/day and 360 days/year. Motor efficiency = 0.698; hypoid reducer efficiency = 0.85; worm-worm reducer efficiency = 0.44. As evident in the graphic, hypoid gears nearly halve utility costs.

Durability

More-durable gearboxes reduce maintenance and replacement costs. Over a period of several years, users will also realize total cost of ownership benefits by switching to a gearbox that may initially be more expensive but has zero maintenance costs. For instance, hypoid gearboxes offer the option of maintenance-free grease lubrication. The Operational costs table shows a typical comparison.

Lubrication

Oil churning, seal drag, and friction account for most of the efficiency losses in gearboxes. Lubrication affects all three to some extent. Seals ride on a thin oil lubricant film. Gearbox components moving through the oil sump cause churning losses.

The lubricant alone can boost gearbox efficiency. Efficiency generally requires the thinnest oil that provides adequate film thickness. And one that contains a good antiwear or extreme-pressure additive package for 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, higher efficiency reduces operating temperatures. The rule of thumb is that for every 10°C drop in temperature, lubricant life doubles.

According to the U.S. Energy Information Administration, the U.S. generated 1,006 billion kWh of electricity in 2007. And it is generally accepted that electrical motors account for about 70% of industrial electric-power consumption. If those electric motors are all driving gearboxes, every 1% improvement 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 modifications to existing equipment.

Safety

For safety-critical applications such as elevators and hoists, engineers typically prefer hypoid gears over worm and bevel gears. Because the hypoid has more contact area, it is more durable 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 significantly wear over time.

In instances where a gear eventually wears, a gear tooth can break unexpectedly and cause catastrophic failure. All-steel hypoid gear teeth will not markedly wear if adequately lubricated - and in the worst case will generate rumbling and grinding noises as a warning that the gearbox is about to fail. Backlash will always worsen for worm gearboxes from wear in bronze gearing. In addition, special lubrication is a must for worm gears, as some lubricants or additives will attack and weaken bronze gears.

This is not to say that worm gears do not have advantages. Higher-ratio worm gear configurations in which the gear cannot drive the worm are said to be self-locking. It is a safety mechanism 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.

Practical considerations

Gearbox manufacturers can maximize efficiency through a variety of means. High-quality gearing with a high-grade surface finish on the gear teeth - combined with low-friction seals, bearings, and lubricants - all make enclosed gearing products more effficient.

To the user, however, the most important factor in selecting a gearbox is whether or not the size is optimized for the application. Size and weight restrictions may dictate shaft arrangements or the gearbox brand. And there are problems if a gearbox is unnecessarily oversized - specifically, if the power capacity of the gearbox greatly exceeds the applied motor power combined with the application service factor. Here, much of the motor power will go to overcome the constant losses within the gearbox, leaving little additional usable power and torque for the application itself. In short, this situation would yield a low efficiency speed reducer. Conversely, a gearbox undersized for an application runs the risk of wearing out quickly from overload conditions despite a seemingly high efficiency.

Many factors affect efficiency. The durability and longevity of hypoid gears often make it possible to replace worm gearbox/motor combination with a smaller-horsepower input-hypoid and actually produce more torque at the driven shaft. In addition, hypoid gearboxes offer the option of grease lubrication that can eliminate oil leaks and make operations cleaner, more environmentally friendly, and maintenance free. Thus in many cases, it makes sense to deworm your operation and replace it with hypoid gearboxes for better efficiency.

TYPICAL EFFICIENCY

Efficiency for hypoid gearing holds relatively constant, and shows significant advantages for ratios greater than 30:1.

Paying a bit more up front for a more-durable gearbox can generate substantial savings in operating and maintenance costs.