rexnord.com | How To Specify & Select Gear Drives

Gear drives are the "lifeline" in most processing operations. And, with today's new emphasis on operating efficiency, selection of the proper drive has become increasingly important. The right choice can result in a big difference in productivity, operating costs, and energy savings.

Selecting the right gear drive ideally brings together the gear manufacturer, the system designer, and the end user. At the outset, a gear manufacturer must know what the drive will be used for, the demands to be placed upon it, and the nature of the equipment it will be driving.

Similarly, the user and the system designer must be familiar with the variables that affect performance and service. For example, an application that places a torque load on a drive in excess of its rated capacity will inevitably result in tooth surface distress and, in severe cases, breakage. Tooth surfaces that show signs of wear or pitting should be candidates for future preventive maintenance programs. Fracture of a gear tooth will not only put the gear drive out of service, but could possibly do damage to bearings and shafts. A number of factors enter into the selection of a gear drive, including service factor, drive rating, thermal capacity, speed variation, equivalent horsepower, drive ratio, and physical size. All must be carefully evaluated to make the right decision.

Service Factor

"Service Factor" (SF) combines external load dynamics, reliability, and life and is used to calculate equivalent horsepower. Acceptable values of Service Factor for different applications are determined by field experience. For example, the American Gear Manufacturer's (AGMA) practice for enclosed speed reducers contains a listing of applications with their proper service factors.

A SF value between 1.25 and 2.0 is typically chosen and then multiplied by the motor nameplate power to establish that required by the driven equipment. Gear drives must be sized so that the peaks of the running load do not exceed the endurance limits of the components.

Gear drives, supplied in combination with electric motors, may be designated with a "service class number" such as I, II, or III rather than a numerical SF. Class I, II, or III are equivalent to SF values of 1.0, 1.41, or 2.0. Service class and service factor can be used interchangeably.

However, numerical designations are preferred because service class does not accommodate intermediate values of SF.

Published service factors are only "the minimum recommended," for a given application. Applications involving unusual or severe loading, or those requiring a higher degree of dependability, should be reviewed with a gear manufacturer. Typical values of SF will not accommodate systems that have serious critical vibrations, or repetitive shock loading. These will require changes to be made in the inertia or spring constants of the system. The responsibility for identifying vibratory loading prior to gear drive selection must rest with the system designer.

Gear Drive Rating

Published ratings of a gear drive are determined by the mechanical load-carrying capacity of gear tooth elements, rotating shafts, and bearings. The AGMA has adopted standard practices for establishing gear drive ratings.

The horsepower rating of a gear tooth is less than or equal to the durability (pitting resistance) of the surface, or strength (bending fatigue) rating as determined by established AGMA criteria. The relationship between gear life (based on pitting resistance) and load, is proportional to the increase in SF raised to the 8.78 power. For example, if SF is increased by 30 percent, the gear tooth life will increase 10 times.

Shafts support the gear tooth elements transmitting torque from the motor to the driven machine and also distribute the radial loads to the bearings. While shafts are designed for carrying torsional and bending stresses, they minimize deflection by maintaining uniform contact across the gear face.

Rolling element bearings are selected according to bearing manufacturers' recommendations. They are based on transmitting the rated horsepower at an SF of 1.0 for less than 25,000 hours average expected life. Bearing life is defined as the number of hours of operation at a constant speed before the first evidence of fatigue develops on either the raceway or rolling elements.

The designation for average life is L-50, which is equivalent to a minimum L-10 life of 5,000 hours with 90 percent survival. The relationship between bearing life and load is proportional to the increase in SF raised to the 3.33 power. For example, if the SF of a gear drive is increased by 30 percent, the bearing life will increase 240 percent.

Determining Thermal Capacity

Thermal capacity is the amount of horsepower a gear drive can transmit continuously for three hours or more without exceeding an operating oil sump temperature rise of 38°C (100°F) above ambient (The maximum acceptable temperature for an oil sump is 93°C (200°F)). Thermal capacity can limit selection of a drive when it is less than the nameplate rating of the motor if there is no auxiliary cooling. SF is not involved since heat dissipation is based upon average power consumed - not peak loads.

Helical gears supported by rolling element bearings operate with the highest efficiency of any major power transmission system. The power losses are usually less than 1.5 percent per gear mesh. Figure 1 shows the distribution of power losses within the gear drive. Churning loss results from gear blanks revolving in the oil sump and generating splash for lubricating the gear teeth, bearings, and seals. These losses are a function of speed and oil level, and are constant regardless of the power transmitted. This is significant because under a light-load, full-speed test, measured efficiency will be less than predicted.

Ambient temperature degrees C (F)	Thermal horsepower multiplier
10 (50) 24 (75) 38 (100) 52 (125)	1.40 1.20 1.00 0.75

Gear drives are designed with a variety of internal hardware to minimize losses, while still assuring adequate splash lubrication. These include: oil exclusion pans to reduce churning, wipers to collect oil from the rotating gear for distribution to the bearings, and dams to maintain a reservoir at the bearing. To assure proper oil distribution at very slow speeds, higher oil levels are used. Oil levels in excess of recommended, results in greater churning losses, increased power consumption, and higher operating temperatures.

Checking the thermal capacity of a gear drive is extremely important. If the unit generates heat faster than it can be dissipated, loss of operating life or severe damage can occur.

This may take the form of surface distress on the gear teeth or hardening of the oil seals, resulting in leakage. Figure 2 shows the relationship of the oil sump temperature to the speed of the driven equipment. Reducing operating temperatures will increase the oil film thickness at the gear teeth and bearings, and will increase the life of the equipment.

Heat is generated by a gear drive through frictional losses. The gear lubricant is the carrier of this heat, which is then distributed to the housing and conducted to the outside surface, where it is dissipated. If the thermal capacity of the gear drive is greater than the motor nameplate rating, and the ambient temperature is below 38°C (100°F) in the immediate vicinity of the drive, you can expect an operating sump temperature less than 93°C (200°F). If, on the other hand, the drive is in a very confined area, and is coated with dirt or waste material, a high probability of distress can be expected, as shown in Figure 2. A corresponding shorter operating life can also be expected.

Figure 2: Probability of gear distress based on operating temperature of the oil sump.

Effect of Speed Variations

Variable speed applications fall into two load categories: constant torque or constant horsepower. Constant torque occurs when load demand varies proportionally with a change in speed. Gear drives are basically constant torque machines requiring no selection modifications. If the application is a constant horsepower one (load demand is constant regardless of speed), the gear drive must be selected for the slowest speed at which the motor will deliver its rated horsepower capacity. This must also be done when a mechanical, electrical, or hydraulic speed reduction device is used between a gear drive and constant AC motor. Variable or multi-speed applications also require special considerations to provide adequate splash lubrication at the slowest speed, without excessive heating or churning at the higher speed.

Determining Equivalent Horsepower

Gear drives are selected based on the nameplate capacity of the driving electric motor multiplied by the mechanical SF reflecting the operating characteristics of the driven load. The gear drive must be compatible with the motor to meet changing conditions, and in the case of variable speed applications, to the motor rating at base speed.

Frequently, electric motors are purchased with an additional 10 or 15 percent capacity. This should not be confused with the mechanical SF used to obtain the equivalent horsepower. The motor electrical service factor must be considered independently in evaluating actual transmitted hp. For example, a 74.6 Kw (100 horsepower) motor with a 1.15 service factor will deliver 74.6 kW (115 horsepower) continuously. Selection of the gear drive would be on the basis of the 74.6 kW (115 horsepower).

Finding The Ratio

To arrive at the specific gear ratio required, divide the motor full-load speed by the rpm of the driven equipment. AGMA nominal ratios are based upon the geometric numerical progression of the square root of 1.50. Exact ratios are determined by dividing the actual number of gear teeth by the mating pinion teeth - both of which are whole numbers. Deviation between AGMA nominal and exact ratios are +/- 3 percent for a single reduction gear drive, and +/- 4 percent for a double reduction.

For applications driven by variable speed DC equipment, exact gear ratios become less important. In that case, it is best to select a manufacturer's standard ratios. These will not only provide lowest cost and shorter delivery, but offer ready, off-the-shelf stock spare parts.

Choosing The Right Size

Manufacturers' catalogs provide input speed, ratio, and kilowatt rating for use in determining the size of the drive. But before the purchase order is issued, there are other factors to consider. These include: type of unit, initial cost vs. the cost of maintenance, useful operating life, and spare parts if a marginal selection is made. For example, a 30 percent increase in initial cost by specifying a gear drive one frame size larger could easily represent a 240 percent greater bearing life and 10 times greater gear tooth life. Such alternatives must be weighed by the systems engineer. Gear drives are available in a variety of sizes with various shaft configurations to meet your space requirement. The most popular are: parallel shaft, concentric, and right angle, with either low-speed shaft horizontal or vertical to the input shaft centerline.

Under normal circumstances, reliability is evaluated as part of the SF which accounts for the effect of the normal statistical distribution of failures found in material testing. Gear teeth designed to AGMA standards are based upon a statistical probability of fewer than one failure in 100. If your experiences show that an SF of 1.25 has given satisfactory service in the past with normal maintenance, the identical gear drive should be used on new purchases. If, on the other hand, you have a new, yet-to-be-proved application, a more appropriate reliability factor may be one failure in 1,000 or even one in 10,000.

It is difficult to estimate the operating life of a gear drive. Gear teeth designed to AGMA standards are based on 10 million tooth contact cycles. Figure 3 shows the effect of increased bearing life when higher-than-normal service factors are used. Designing in accordance with AGMA standards requires that all anti-friction bearings be selected for a minimum of 25,000 average life hours. L-10 is one-fifth of this value. The simultaneous occurrence of the worst condition of kilowatt, speed, ratio and rotation, with full catalog overhung load, would have to occur before the minimum 5,000-hour L-10 life would be reached. Most designers recognize that a higher-than-minimum SF is cheap insurance compared to costly downtime when a process is interrupted due to the failure of a single component. See Fig. 3.

Figure 3: Bearing life of a gear drive as a function of SF for 24-hour-per-day operation.

Once you've selected your gear drive and put it to work, it is penny-wise and dollar-foolish to operate it under marginal thermal conditions. Basic to long-life operation is, of course, maintaining the proper level of oil in the sump, or a steady supply of cool, filtered lubricant. Factors that can affect performance and wear, such as operation in an elevated temperature, or a high oil level, can be detected by a consistent preventive maintenance program followed by immediate corrective action. For example, a change in the viscosity of the lubricant may arrest initial wear or gear pitting. A better alternative would be to lower the overall operating temperature by using auxiliary cooling.

To increase the thermal capacity, shaft-driven fans can be mounted on the high-speed shaft of the drive. This forces air along the exterior of the housing to provide increased convection losses in the area of greatest heat generation. The fan should be sized to provide optimum air flow with a minimum of turbulence to the surrounding area. In this way, the thermal capacity of the unit can be increased up to 2.5 times the catalog thermal rating. Safety regulations dictate the size of the openings in the fan guard, which must be kept clear to maintain effective cooling.

Commercial air-to-oil radiators are also available to increase thermal capacity. They are best used in a remote location where the ambient temperature is lower than that in the vicinity of the gear drive. Circulating cool, filtered oil through the gear box substantially increases the operating life of a gear drive. Circulating systems can be added to each drive; or where units are grouped in series, a central system can be used.

With central cooling, the gear drive maintains its reliable splash lubrication system, while small quantities of clean oil supplied under a nominal pressure are circulated through the unit.

The excess flows out of an elevated drain at the operating oil level to a common return line which links all units in the drive system. The returned oil is allowed to settle in a special tank with baffles to interrupt flow before the suction side of the main pump moves it through the filter and into the distribution network.