Motion System Design
Gears and bearings rise to auto plant challenges

Gears and bearings rise to auto plant challenges

As auto manufacturers strive to improve their products and satisfy customers, advances in gears and bearings do their part in upgrading the performance of auto manufacturing machines on the plant floor

To compete more effectively in a global market, U.S. automobile and truck manufacturers need manufacturing equipment on the plant floor that runs faster, more accurately, more reliably, with less maintenance — and the list goes on.

Though gears and bearings are essentially mature products, their advances play an important role in helping machine manufacturers upgrade their products, and auto manufacturers to use them more effectively. Lets look at some examples.

Gear capacity and efficiency

Conveyors rely on gear drives in performing their many tasks throughout manufacturing plants. Auto makers want these drives to crank out more torque in a smaller package, and to last longer, notes Bob Bruzina, district manager in Detroit for The Falk Corp.

So gear manufacturers refine tooth geometry and use hardened and ground gear teeth to boost capacity and life. Case hardened teeth, especially carburized, are becoming more prevalent. Right-angle and concentric-shaft gearboxes with these high-capacity gears are said to deliver 30 to 50% more torque in floor, overhead, and indexing conveyors.

But, generating more torque in the same envelope often has a downside — more heat buildup — so external cooling may be needed to help cool the gears and bearings.

One machine builder, ConveyorMatic builds floor conveyors for plant-wide operations. Fred Kysia, general manager of engineering, comments that U.S. auto manufacturers produce better vehicles than a few years ago. Part of their success lies with machines that run faster and last longer, due to better gears and other components. Whereas a typical plant produced 40 cars per hour 10 years ago, the number is now approaching 80.

Efficiency. To get more machine efficiency, auto manufacturers are moving away from worm-gear speed reducers to helical gears and helical-bevels, according to Allen Kornow, field sales engineer, Rockwell Automation/Dodge. His company supplies speed reducers for floor conveyors, such as large skids that transport car bodies from point to point. This switch in gear types generally nets at least an 8% gain in gear efficiency.

Bearings boost accuracy

Many auto plants demand tighter tolerances on strip steel, which is produced in rolling mills and used to form sections of the auto body or “skin.” William Meese, product manager, large industrial bearings, The Timken Co., believes the squeeze on tolerances began with the quest for better gas mileage, which calls for smooth aerodynamic shapes. Most important, consistent thickness prevents unsightly ripples in the formed part.

Strip thickness variation is affected by several factors, including the runout of bearings in the large backup rolls that support the work rolls. This runout depends on dimensional variations in outer race, inner race, and rolling-elements.

Precision roll-neck bearings reduce runout, thereby shrinking the strip thickness variation. This improves body panel quality, saves material, and reduces cost, says Mark Baker, senior application engineer for Timken. A recent improvement in grinding bearing races makes it possible to cut their dimensional variation 25%, which further reduces runout. However, the company will evaluate this process further before offering these more precise bearings.

High-speed bearings

Today’s machine tools are expected to operate faster, but with less vibration, which affects work piece quality. High-speed precision bearings help to meet these requirements in milling and boring machines for engines and transmissions, according to Matt Boylan, business manager, Fafnir Div. of The Torrington Co.

A modified bearing geometry boosts the speed capability by 20 to 25%. Smaller-size balls are used, which means lower centrifugal force. Also, a more open race curvature (larger radius) reduces the contact area between race and roller. Smaller contact means less friction, ball skidding, and heat generation, which also enables higher speed.

Because the balls are small, there are more of them, which increases bearing stiffness typically by about 12%. Stiffer bearings rigidize a machine tool spindle, an important factor in limiting vibration. If a spindle is not sufficiently rigid, vibration adversely affects the surface finish of the end product. It also causes premature tool wear, shorter spindle life, and reduced machine speeds (and productivity).

“The bearing is one of the critical components in the spindle head,” says Lawrence Hermanowski, product manager at spindle-manufacturer Whitnon Inc. “Rigidity must be maintained for the process to be vibration free.”

Most spindles have ball bearings. But tapered roller bearings are often used in the spindles of auto manufacturing machines because of their higher load capacity and stiffness according to John Chadwick, principal application engineer at Timken. With these capabilities, you can often replace two or three rows of balls at each end of the spindle with a single- row tapered bearing.

Ceramic balls. Some high-speed bearings contain ceramic balls, which typically have better finish; can operate with marginal lubrication; and are 40% lighter than steel balls. With the lighter ceramic balls, centrifugal force is less so the bearing can go faster. For example, a bearing with ceramic balls can generally operate 20 to 40% faster than standard bearings with steel balls.

Ceramic balls generate less heat, so you can switch from oil lubrication to grease without the usual heat buildup at high speed. For this reason, bearings with ceramic balls are used in high-speed grinders and milling machines.

Ceramic balls are harder than steel balls (70 Rc vs. 60 Rc), which reduces wear. And the chemically inert ceramic material (silicon nitride) protects rolling elements from corrosion caused by cutting fluids.

Bryant Grinder Corp., maker of CNC grinding equipment and high-speed spindles, began using bearings with ceramic balls in 1988, says Dr. Russell Kulas, manager of spindle technology. These bearings provide a marked improvement in vibration — mainly by minimizing defects in balls and raceways.

“Reduced heat from the ceramic balls allows better control of material expansion, and that means better accuracy, adds Mr. Hermanowski of Whitnon Inc. “Also, the lubricant doesn’t thin out, and therefore remains effective longer.”

Self-lubricating bearings

Like many others, auto manufacturers have cut their labor force to compete in global markets. According to Bob Schroeder, president, Pacific Bearing Co., one effect of this cutback is that personnel (with less time or training?) either neglect or over-lubricate bearings. Both conditions shorten bearing life. With over-lubrication, excess grease breaks through the seals, letting contaminants enter the bearing.

Contamination is especially harmful in welding applications, adds Ray Stojonic, manufacturer’s rep and sales engineer for Pacific Bearing. Besides causing bearing wear, weld dust clogs the ball paths of recirculating bearings and stops the ball motion.

There are a couple of ways to minimize such problems, yet eliminate the need for periodic lubrication:

• Self-lubricating plain bearings that can be used for either linear or rotary oscillating motion (less than 360 deg). Such bearings are often used in welding machines and transfer tables.

• Encapsulated oil in a jell-like matrix of UHMW and PTFE polymers can be placed inside rolling-element bearings. The matrix surrounds and lubricates the rolling elements while keeping contaminants out. This lubricant is well-suited for overhead conveyors because, unlike excess grease, it doesn’t fall on the floor or the end product.

Another effect of reduced labor is a demand for machines to perform functions previously done by humans. Unlike older equipment, such machines are expected to maintain accuracy throughout their life.

In many cases, older machines no longer produce quality parts. A welding robot may loose its positioning accuracy because of excessive play in worn rolling-element bearings. In some cases, substituting a onepiece, self-lubricating bearing may reduce such wear and loss of accuracy.

Where to from here? Overhead monorail conveyors in auto plants typically have two parallel rails or linear guide ways with recirculating ball bearings. These systems present two challenges for the future: how to carry heavy loads without failure, and how to avoid the close mounting tolerances required to keep the two rails aligned?

One possible approach, if the machine configuration permits, is to substitute self-lubricating plain bearings for the ball-bearing system. The line or area-contact of these bearings provides more load capacity than a comparable ball bearing, which has point contact.

A future driving force is in the making with the North American Automotive Metric Standards (NAMS) being jointly developed by GM, Ford, and Chrysler. Its purpose is to develop standard manufacturing system specifications so that vendors can offer identical components to different auto companies. Lubrication Auto manufacturers can’t afford downtime, yet they don’t want to perform lubrication or maintenance. So they look for alternatives, such as components that are lubricated for life.

Oil-mist lubrication systems are beginning to lose favor because they tend to cause environmental pollution. As an option, some plants prefer to grease bearings because the system is self-contained and doesn’t require supply lines as do oil lubrication systems. The tradeoff is that grease limits speed.

Prolonging the life of a lubricant usually calls for a synthesized (highly refined) petroleum-based lubricant that resists breakdown, says Marco DeMaria, product engineer, Rockwell Automation/ Dodge. Although such lubricants are more expensive, their life is said to be four times that of conventional types.

Contamination is a big factor, especially where bearings are exposed to hydraulic fluid or welding dust. Improved seals are the best weapon against contamination. This generally means adding lip seals, flingers, or labyrinth seals to individual bearings; and possibly labyrinth, V-ring, or (trash guarding) flinger seals in the bearing housing (pillow block).

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