Don’t bust that BALLSCREW

Oct. 1, 2000
No one plans to shorten the life of a ballscrew, or damage the ball nuts, or bend the screw. But things happen that can usually be traced back to lubrication, load, or operating speed. To reduce the potential for damage, here are the ten most common problems you should watch for and techniques you can use to prevent them.

Running dry

Lubrication reduces friction, minimizes heat generation, and retards wear and corrosion. Lack of it is the most common cause of ballscrew problems. In fact, operating without oil or grease can almost immediately destroy the screw.

The shaft should be kept damp with the lubricant that best matches the application. Each type of grease and oil reacts differently to temperature and other environmental conditions based on their additives. For most applications, such as those with operating temperatures from 0 to 180° F, you can use a good grade mineral oil for rolling element bearings. For those applications requiring grease, use a good NLGI Grade 2 bearing grease with EP additives. Other types of greases and oils are available for temperatures above or below this range. The ballscrew manufacturer can guide your selection.

The type of lubricant selected affects the application frequency. In addition, many lubricants indicate if the frequency must change. For example, if your lubricant is changing color or evaporating, then apply it more often.

The orientation of the ballscrew will also affect lubrication. For example, on vertically mounted screws, use methods to ensure the lubricant stays evenly distributed.

Lack of support

Both fixed and supported ballscrews need adequate bearing support because speed and length affect how the screw reacts to the load. To prevent damage to either the screw or the ball bearings, size the support bearings according to the manufacturer specifications. Be aware, though, that problems can occur through stress risers at journal diameters. So use generous radii and correct bearing fits.

Bearing systems should reduce stress concentrations created by pulleys, gears, and belts. These drive systems tend to bend the end of the screw shaft, fatiguing it back and forth as it rotates. If not compensated, the stresses will eventually ruin the screw.

A shocking experience

Sudden load changes happen in manufacturing, but such impact and shock loads are detrimental to the ballscrew ball bearings. They create forces that can cause the balls to run roughly, bring about race brinneling, or even fracture the balls. If shocks may occur in the application, be sure to provide adequate dampening.

The long and short of it

There's another load that if ignored can damage a ballscrew. It's known as the maximum compression load and is the ability of a ball screw to avoid buckling under this load. It is a function of the square of the length of the screw, it is usually much lower than the screw material's compression strength. Thus, the longer the shaft, the more prone it is to buckling, so the screw's column strength is often the controlling design parameter.

In addition to being a function of length, compressive load strength is also a function of the cross-sectional moment of inertia and end-fixing. Follow the manufacturer's guidelines when specifying the length of the ballscrew or put long columns in tension.

Get it straight

Attention to detail is crucial to proper ballscrew mounting. Even if the installation is off by "just a hair," it will shorten ballscrew life. Any misalignment among the ball nut, screw, bearings, and mounting surfaces can create internal forces that increase wear or cause excessive heat or vibration.

To properly align a screw, first, always be sure to support it on its outside diameter. The ball nut should never support the weight of the assembly. Remember: you need to react loads through a pure axial thrust direction.

Next, assemble the support bearing to the ballscrew and place this assembly on the mounting bed. Fasten the bearing support at the drive end to the bed, but only hand tighten the bolts. Then fasten the nut to the carriage or cross-slide and hand tighten those bolts.

Manually rotate the ballscrew, moving the carriage to the drive end. Now fasten the nut securely, tightening bolts to the manufacturer's recommended torque.

Too much to bear

A good way to damage a ballscrew is to make it move an excessively heavy load. How much is too much? To determine that, you need to consider two factors. Dynamic capacity is the rated axial load that can be applied during linear travel for a given life expectancy. Static capacity is the maximum axial load. Most manufacturers' catalogs offer load ratings or calculation information.

In some cases, you can exceed dynamic capacity. But the ballscrew will probably have a shorter life because the extra load may accelerate fatigue or the plastic deformation of the balls and raceways.

If you exceed static capacity, the load will permanently deform the raceway track. If ballscrew operation is rough and noisy, this may be the reason.

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With both capacities, the worst case scenario of high loads is that the materials yield or exceed their compressive limits and the ballscrew severely deforms or even breaks.

Neither too hot nor too cold

Even though ballscrews can handle the typical manufacturing environment, extreme temperatures may cause problems. Temperatures over 300° F can reduce the raceway hardness. Temperatures below -40° F can reduce ductility, leaving the screw brittle. The lead advances of the screw will also change when exposed to such temperature gradients.

For such applications, try a ballscrew made of stainless steel, alloys, or with special coatings designed for harsh environments. Also, don't forget to use lubricants made for temperature extremes. Some lubricants will inhibit corrosion too. Another option is to use bellows, wipers, and guards. These devices will protect the screw against dirt and contamination.

Speed maims

Each screw has a maximum operating speed that you shouldn't exceed. As high rotational speeds reach the natural frequency of the screw, they cause it to vibrate and whip. These vibrations distress the ballscrew assembly, which can increase the rate of wear, lower positioning accuracy, and fatigue the balls and raceways.

If you need a specific feed rate, you can use a higher lead advance and reduce the screw speed rate. This is one way to control critical speed yet still achieve the same feed rate. A stiffer bearing arrangement with spaced dualangular contact bearings at each end of the ballscrew will also help.

None on the side

Loads should always move along the axis of the ballscrew shaft. Even a low side load or moment load can shift the balls in the nut, which compromises their load-carrying capacity. Such loads can also cause faster wear, excessive heat, ball scuffing, and ball lockup.

If the side or moment load is more than 5% to 10% of the axial thrust, consider supporting the load with an additional system, such as linear guides or linear bearings.

Put it away

If you're not planning to use a ballscrew upon delivery, then be sure to store it properly. It should be supported by its ends in a horizontal or vertical position and protected from the elements. For horizontal storage, after carefully unpacking the ballscrew, set it on vee blocks. Place the supports close enough to eliminate sag. For vertical storage, simply hang it by one end.

If the ballscrew will be stored for several months or longer, then be sure to reapply a rust preventative often. Otherwise you may retrieve your ballscrew from storage only to find rust damage because the preventative did not last.

Jim Babinski is manager, product engineering for Thomson Saginaw Ball Screw Co., Saginaw, Mich. He is also Chair of the Technical Committee 43-Ball Screws, of The American Society of Mechanical Engineers (ASME).

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