BEARING THE BRUNT

In these tough times, nothing escapes attention in the automotive industry's relentless quest for efficiency and fuel economy, not even the amount of energy required to overcome the internal friction of a ball or roller bearing. In fact, several bearing manufacturers now offer a line of low-torque bearings aimed specifically at this and other energy-sensitive markets, claiming improved fuel economy, reduced weight, noise, and operating temperature.

But the impact of bearing torque doesn't stop there, because that same pursuit of efficiency is now a hallmark across all of manufacturing due to the continuing sensitivity of global energy costs. For example, machine tool bearings used in the spindle, rotary table and ball screw must also deliver long life, tolerate heavy loads, and maintain extreme precision or stiffness. This means the precise, efficient measurement of bearing torque is a critical aspect of machine tool construction.

But is it possible to precisely and efficiently measure bearing torque?

Bearing torque is the force required to overcome both static and dynamic frictions between the inner and outer rings of a bearing. This torque is influenced by a number of factors, including bearing size, load, speed, and the tolerances of the various components. There are three separate measures of bearing torque: starting or breakaway, average running, and peak running (see Figure 1):

? Starting torque (break-away): Restraining torque that must be overcome to start rotation of one ring of the bearing while the other remains stationary.

? Average running torque: Mean torque encountered during rotation of at least one complete revolution. Average torque values provide a measure of consistency within a group of bearings.

? Peak running torque: Maximum torque encountered during rotation of at least one complete revolution. Peak torque values provide a measure of quality within a group of bearings.

All three values are typically measured with the bearing under load using a standardized oil lubricant. Obviously, these values can be quite small, as low 0.002 Nm for some miniature precision bearings. Bearings are, after all, anti-friction devices and modern bearings are produced to extremely tight tolerances (see Figure 2).

Lubrication type greatly influences bearing torque. Oil-lubricated bearings generally have lower low-speed torque than grease-lubricated bearings. However, grease-lubricated bearings may have lower torque at high speeds, particularly if channeling grease is used. High-viscosity oils have a higher torque than low-viscosity oils.

Bearing design also influences torque. Lightweight metal and molded plastic retainers have the least torque for low and moderate speeds. Phenolic and sintered nylon retainers have lower torque at high speed. Bearings with very low contact-angle or radial-play values show high torque because inherent geometrical errors in the bearing raceway and balls cause erratic variation in stress and, hence, friction level. Very-high contact angles create higher torque levels because of the sliding motion in the ball-to-race contact area.

The torque measurement must be done in a way that only the friction torque of the bearing is measured. This can be done by eliminating any "fixture" torque from the measured value. The challenge is in the frictionless fixturing of the bearing. One solution to achieve an accurate measurement of the bearing friction is an air table with integrated torque sensing (see Figure 3).

An instrumented air table provides a near-zero friction torque solution. The preload of the bearing is carried by the air table where the friction of the air cushion is so small that it can be ignored. All bearings are normally pre-loaded with a defined load that is dependent on size and use of the bearing.

Defining the correct pre-load is especially critical when testing tapered roller bearings. If the right preload of a bearing is not known, an ultra precision torque measuring station will allow for optimization of preload by applying various loads and measuring the change in frictional torque of the bearing.

This station is developed to measure break-away torque, running torque and peak torque on bearings with programmable axial pre-load and programmable rotation speed. It has been designed as a diagnostic tool capable of measuring torque across a wide array of bearings ranging from miniature ball bearings to very large, heavy-duty tapered roller bearings.

Because the station can quickly test a bearing under a variety of different load and speed combinations, it becomes a relatively simple operation to empirically determine the optimum pre-load for any given set of operating conditions.

The three major mechanical components of the Torque Stations are:

? The Rotational Electro-Mechanical Assembly Press (REMAP)

? The Air-Table

? The Work Station

REMAP is an off-the-shelf solution for the pre-load and rotational aspects of a low-cost, standardized bearing torque measuring system (see Figure 4). The REMAP consists of an electro-mechanical assembly press (EMAP) combined in a single unit with a torque functional test (TFT) unit. Together, they produce a system that provides independent, programmable control of both linear and rotational motion using two separate servo axes.

Included in the TFT is a strain gauge-type torque transducer between the motor and the shaft. This transducer consists of thin electrical conductors attached to a spring-like structure. As the structure deforms under torque the minute changes produced in the conductors alters their electrical resistance in direct proportion to the amount of torque applied.

Both devices are controlled by an electro-mechanical multi-axis controller (EMAC), an easily programmed, fully integrated, multi-axis motion controller, data acquisition and analysis system. The EMAC tells the system what to do and when to do it and then gathers and analyzes the results using application software developed specifically for bearing torque measurement (see Figure 5).

Additionally, the air table with integrated torque sensing helps meet the need for robust, flexible, frictionless fixtures. The instrumented air table provides near-zero friction even while accommodating the high pre-loads required for testing large ball and roller bearings. All of these components can be packaged inside a standard workstation, a robust, standardized unit that houses the REMAP, EMAC, air table, an operator interface with LCD display, and an electrical cabinet (see Figure 6).

After loading a bearing into the gauge and initiating a cycle start, the torque tester executes a fully automatic test program. Once completed it displays the pre-load, speed, break-away torque, average torque, peak torque and the torque vs. angle graph.

Technical Data:

? Torque Measuring Ranges: 0 – 0.1, 1, 2, 5, 20, 50 Nm (0 – 0.9, 9, 18, 50, 180, 450 in/lb)

? Accuracy: <1.5 percent of Full Range

? Measuring capability down to: 0.002 Nm (0.018 in/lb)

? Axial Load Range: up to 12,000 N (2740 lb)

The precise, efficient measurement of bearing torque is becoming a critical aspect of machine tool construction as global energy costs continue their inevitable rise. These measurements can be facilitated by the technologies packaged in this type of ultra precision torque measuring station.

Larry Stockline is the president of Promess, Inc., 11429 Grand River Road, P.O. Box 748, Brighton, MI 48116-9547, 810-229-9334, Fax: 810-229-8125, www.promessinc.com.