Maintaining And Increasing The Life Of A Ball Screw

Increasing the Life of Ball Bearing Lead Screws

Ball screws are more easily damaged when installed than conventional lead screws. To help prevent damage, ensure accuracy, and longer life; proper installation is required. Barnes Industries ball screws are manufactured, inspected, and tested by skilled mechanics to engineered specifications, ready for installation.

Engineering of Ball Screws

Ball screws are super high precision, preloaded ball bearings running on a helical raceway. To build a new unit, or recondition a unit to new high quality condition, extensive ball screw engineering analysis is a basic requirement.

Ball screw life can be calculated. Parameters used in calculations include contact angle, ball conformity to the thread radius, number of balls in contact with the raceway, preload, materials, heat treatment case depth and raceway hardness. Manufacturing control of parallelism, squareness, concentricity, and lead are also necessary to achieve ball screw accuracy and long wear life.

Since the formation of the company, Barnes Industries engineers have analyzed many failures on major U.S. and foreign ball screws. To prevent failures and give extended life, designs have been continually improved, manufacturing tolerances reduced, and design recommendations made to customers.

Engineering analysis of failed ball screws since the formation of Barnes Industries has permitted "Barnes" to establish engineering and manufacturing specifications in which 98% reliability has been attained. "Barnes" new ball screws are warranted for 24 months or 20 million inches of nut travel.  Reconditioned ball screw warranties fall into two categories (1) 18 months or 15 million inches of nut travel (2) 15 months or 12.5 million inches of nut travel. See specifications for each of the services provided for complete details.

Load and Life Rating

Ball screw life ratings for ball lead screws are comparable to the L-10 rating for ball bearings as determined by the American Bearing Manufacturers Association. The L-10 or B-10 rating is defined as a reliability requirement * where a failure rate of 10% is allowable. This means that about 90% of any large group of identical screws will meet or exceed this life rating, 50% will last five times as long, and about 10% will fail before reaching that rating.

 

*BALL SCREW LIFE CHART FOR BALL NUTS WITH L-10 RATING DESIGNED FOR 20,000,000 INCHES TRAVEL FOR A REVERSING LOAD.

Out of 100 Identical Units	Ball Nut Carries Load In Both Directions, Nut Travel (Inches)	Hours Life of Ball Nut Traveling at Inches Per Minute
		100 IPM	200 IPM	300 IPM	400 IPM	600 IPM	800 IPM
10	Less Than 20,000,000	NA	NA	NA	NA	NA	NA
90	20,000,000	3,334	1,667	1,111	834	556	417
	40,000,000	6,668	3,334	2,223	1,667	1,111	834
	60,000,000	10,002	5,001	3,334	2,501	1,667	1,250
	80,000,000	13,336	6,668	4,445	3,334	2,223	1,667
	100,000,000+	16,670	8,335	5,557	4,168	2,778	2,084

NOTE: In the case of a vertical screw or jacking load, the figures in the chart would be reduced to one half, since the load is always in one direction on the screw.

 

CHARTS COMPARING LIFE INCREASE BY USING ALL LOAD BALLS IN CONTACT WITH RACEWAYS WHEN ALL OTHER PARAMETERS MEET ENGINEERING STANDARDS.

Out of 100 identical units, 20 units with load and spacer balls are distributed in chart 'A' and 80 units with all load balls are distributed in Chart 'B'

Out of 100 Identical Units	Ball Nut Carries Load In Both Directions, Nut Travel (Inches)	Hours Life of Ball Nut Traveling at Inches Per Minute
		100 IPM	200 IPM	300 IPM	400 IPM	600 IPM	800 IPM
*Chart A
2	Less Than 20,000,000	NA	NA	NA	NA	NA	NA
18	20,000,000	3,334	1,667	1,111	834	556	417
	40,000,000	6,668	3,334	2,223	1,667	1,111	834
	60,000,000	10,002	5,001	3,334	2,501	1,667	1,250
	80,000,000	13,336	6,668	4,445	3,334	2,223	1,667
	100,000,000+	16,670	8,335	5,557	4,168	2,778	2,084
*Chart B
80 (20 units Fall Into Life Chart Above)	160,000,000	26,672	13,336	8,891	6,668	4,445	3,334
	320,000,000	53,334	26,672	17,778	13,336	8,890	6,668
	480,000,000	80,016	40,008	26,672	20,004	13,336	10,002
	640,000,000	106,688	53,344	35,344	26,672	17,781	13,336
	800,000,000	133,360	66,680	44,453	33,340	22,227	16,670

By using all load balls in 80% of the ball screw assemblies, the L-10 life rating is changed to an L-2 rating or 98% reliability. This is demonstrated in the life chart above.

Load and life rating of ball screws are based on axial load. This applies to both the manufacturer and the reconditioner. The figures below show how axial load and radial load affect the life of a ball bearing lead screw.

Fig. 1 - Chipped outside diameter on worn lead-screw resulting from improper ball contact angle and overload condition.

Fig. 2 - Improperly radial-loaded ball screw assembly showing ball contact path in bottom of raceway. This condition reduces life to 1/8 of original.

Fig. 3 - Axially-loaded ball screw assembly showing correct ball contact path on raceway.

Running Condition

A ball bearing lead screw is nearly frictionless in operation. The load between the screw and nut is carried by ball bearings which provide the only contact between the nut and screw. The ball screw assembly will operate with either the nut rotating around the screw or the screw rotating through the nut. A simple check for running may be made in your plant:

1. Make sure the screw is clean and lightly oiled.
2. Rotate the nut around fixed screw by hand. The nut should rotate smoothly with no sticking or hang up. Sticking should not be confused with the balls being squeezed as they enter the raceway going under load between the nut and screw from the return system. Sticking or hang-up (keystoning) causes the balls to skid, wearing flat spots on the balls, and the nut will ultimately lock up. In severe cases, skidding balls will damage screw raceways beyond repair.

Torque and Preload

The torque to drive the load is as important as all other factors related to the design of a ball screw. These factors are inter-related at the original design layout.

High preload provides higher positioning accuracy and high system stiffness, but increases drag torque. Higher than required preload increases drag torque more rapidly than stiffness and results in loss of screw life. A high preload can cause inaccurate positioning. A high preload will also contribute to electrical malfunctions. Low preload gives low system stiffness and low drag torque, resulting in poor system accuracy.

Ball Screw Alignment, Misalignment and Load Application

Close alignment of the ball screw journal mounting with the ball nut is required for long life and positioning accuracy. Radial, bending or overturning loads imposed on the ball screw assembly will reduce the life of the unit and its thrust bearings. Radial ball screw misalignment results in increased torque as the ball nut approaches the bearing mountings. Bending or overturning loads cause rough running and noise. Radial, bending or over-turning loads will have a negative effect on positioning accuracy.

Improper adjustment of the following will overload the ball screw and nut assembly, and contribute to ball screw failure and poor positioning accuracy.

• Gibs or linear roller bearings adjusted too tight or misaligned.
• Counter balances not functioning.
• Air and hydrostatic bearings not functioning.

Ball Screw Maintenance (Lubrication and Cleanliness)

A ball screw must be properly lubricated and kept clean at all times. Otherwise its life will be reduced. When ball screws are not lubricated, the wear life drops by as much as 85-percent. Lubrication reduces friction, prevents corrosion and allows the ball screw to operate more efficiently. Oil and grease are both used for lubrication. Grease is not normally used in low temperature or high speed applications. Graphite grease or grease with suspended solid particles are never used because they tend to clog the ball return system.

The oil supply should insure a light film of clean oil with filtration levels of three (3) microns or less at all times. Lubrication contaminated with dirt and chips increases friction. Balls traveling over metal chips in the ball thread raceway cause "spalling" and the ball screw fails.

Use lubricants recommended by the machine tool manufacturer. As in the case of any high precision assembly, contamination by chips, dirt or other foreign matter will cause and ultimately induce ball screw failure. Limited protection is provided by seals or wipers. Bellows or telescopic covers are recommended if the environment has a high concentration of contaminants. Periodic inspection for cleanliness and lubrication will extend the life of the ball screw.

Storage of Ball Screw Unit

Keep the unit stored in plastic sleeve in the original shipping container. Leave the ball screw assembly in the protective covering until the actual installation in the machine begins. Dirt, dust or chips will settle on the assembly and the wear process of the ball raceways will begin immediately upon operation of the machine. If stored for a long time, turn container over at three month intervals so protective oil will be distributed over the screw.

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