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There are two primary styles of ball spacing technology. The first style uses plastic pieces to separate the balls. The second uses a one-piece plastic retaining belt with ball cups. Both technologies reduce metallic noise and minimize variation of drag force by preventing ball-to-ball contact, with total noise reduction to 5 dB.

Separate plastic spacers prevent ball-to-ball contact. As these spacers are unconnected, they provide better protection against industrial contaminant damage common in harsh environments.

The other style, a one-piece plastic string of ball cups, also prevents ball collision. Each ball cup is connected to the next by a soft strand of plastic. They eliminate metallic noise generated by the steel balls rubbing against a metal retainer; however, contaminants tend to damage the ball-retaining belt, causing failure over time. This is why this type works better in cleaner environments.

Seldom mentioned

Often times there's a false perception that product design is primarily responsible for extending wear or fatigue life. Some designs do boast less ball slippage, thus reducing sliding friction and potential ball wear. However, lubrication and contamination largely determine wear rates. Ask any vendor for test results showing their linear guide or ballscrew product running without lubrication, and data will invariably show that without it wear and failure accelerate, regardless of design. So often, design takes the credit and lubricant does all the work.

Fatigue life

Be careful not to use extended fatigue life results obtained through lubrication methods varied to increase rated dynamic capacities. The life equation for linear guides is defined.

where L equals life, C_a equals dynamic capacity, and P is mean load.

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For example, if a vendor achieves two times the travel distance (or fatigue life) on a linear guide through superior lubrication, would it be fair to multiply L by 2, solve for C_a, and say that dynamic capacity has increased by 1.26 times simply because 1.263 equals 2? No, because fatigue life is extended due to lubrication. No relevant mechanical changes (larger ball diameter, increased number of loadcarrying balls, contact angle) have raised dynamic capacity.

Fatigue life can be increased with improved lubrication or by raising the dynamic capacity. However, dynamic capacity should not be raised or lowered based on the role of lubrication. Rather, dynamic capacity is properly calculated using relevant mechanical features of the product. Fatigue life is properly estimated using the assumptions of adequate lubrication and freedom from contamination. Fatigue life deals with movement under load — how many cycles can be achieved before the rail or slider raceways begin to spall. Caused by impurities, spalling results from subsurface cracks that migrate toward the surface, causing surface chipping. Not to be confused with spalling is pitting, caused by a removal or pulling away of metal and resulting from microwelding surfaces under pressure.

Raceway wear

Let's take a look at wear. Wear life deals with material removal that leads to clearance between rolling elements and raceways. Progressive wearing of raceway surfaces leads to reduced stiffness and accuracy. Under properly lubricated operating conditions, the wear life will be a function of loading cycles, cleanliness, and alignment at installation. Because ball hardness is greater than raceway hardness, raceways wear more than balls.

Ball wear

Though ball wear is an oftenmisunderstood phenomenon, its causes are concrete. Tests indicate that under normal field operation, the diametrical wear amount of steel balls progresses according to the expression.

In one lab test, balls, rails, and sliders were subjected to plastic powder contamination over a distance of 3,000 km. Rails showed four times the wear of balls, while sliders showed eight times as much. Even with contamination, ball wear was minor compared with rail and slider raceway wear.

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