Machinedesign 2453 Standard Setup 200 603 0 0

Taking back stage

June 1, 2003
Rollers rely on lubricants and supporting bearing hardware for smooth operation

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, Ca equals dynamic capacity, and P is mean load.

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 Ca, 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.

Continue on page 2

Action point

When balls come in contact with each other, pressure goes to infinity, and the oil film between them is ruptured — right? No. Under sliding friction the thickness of an oil film is determined.

Let's simplify:

In reality, the ball-to-ball contact area varies directly with load — pressure = load/area. As the ball load increases (as when adjacent balls press harder against each other) the contact area increases. Conversely, as contact area decreases, adjacent balls press more lightly against each other (and load decreases).

Sometimes claims are made that a system's oil films break under a pressure of 3 kg/mm2 as adjacent balls slide against one another. But, achieving this pressure requires ball compression beyond what's possible with simple ball-to-ball contact. As contact area decreases, the load decreases which means stress approaches zero, not infinity. In other words, ball-to-ball contact is always made at a finite area, never a point.

The ability of one ball to press against an adjacent ball is limited. In linear bearings and ball screws the external loads are taken as the balls are compressed between the raceway grooves. As such, ball-to-ball loads are light and result from one ball pushing against another in normal re-circulation.

• Viscosity
Balls roll in raceway grooves while they slide against one another. In general, to establish hydrodynamic lubrication (lubrication associated with sliding) a minimum viscosity of 5cSt (close to water) is required.

Shell Alavania #2 is a grease used in many industrial applications. It has a viscosity of about 100cSt at 100°F. For viscosity to fall to 5cSt, the operating temperature would have to rise to about 300°F. This is well past the melting temperature of plastic ball cages. In almost every case the minimum viscosity requirement is more than met to establish an oil film.

Having met the requirements for proper viscosity (in combination with the minimal load seen in ballto- ball contact) oil films thicken with speed.

While at rest — zero ball speed — oil fills the space below the balls and creeps up the sides of the ball or balls. This produces a thicker layer of oil along the sides. Any motion draws this oil into the area where the balls contact the raceway. As speed increases, the oil film thickens.

Adjacent balls contact each other through sliding friction as they spin in opposite directions. Thus the speed at the area of contact is twice the ball-spinning rate — promoting oil film thickness.

As the balls are harder than the raceways in which they roll, wear patterns and fatigue occur in the raceway surfaces where the load is carried, not from one ball sliding against another.

As the balls spin and turn in different directions, there are no consistent ball-to-ball contacting areas and wear is minimized. Ever notice scratching or scoring wear patterns resulting from adjacent balls sliding against each other? The separation of metal surfaces is caused by the three parameters just outlined — load, viscosity, and speed.

It should be kept in mind that the linear ball speed required to spiral forward (as in a ballscrew application) is approximately ten times higher than the linear ball speed required to move forward in a straight path (as in a linear-bearing application). So ball transfer rates are even higher in ballscrews, thus promoting oil film thickness.

Lubrication fundamentals

The term elasto-hydrodynamic lubrication is used to describe oil film thickness associated with rolling — as when a ball rolls in a raceway groove. It's primarily a function of speed, viscosity, viscosity-pressure coefficients, and load, which equates to pressure. The viscositypressure coefficient is an indication of how fast a lubricant's viscosity increases as pressure is applied. This characteristic varies with lubricant chemistry. As the lubricant is trapped under the load, it approaches its solid state and instantly separates the metal surfaces.

The term hydrodynamic lubrication is used to describe oil film thickness associated with sliding (e.g. as one ball slides against another). It is primarily a function of load, speed, and viscosity.

Oil film thickness cannot be determined by knowing only one parameter. For example, the load is one parameter. But pressure is load divided by area ... so what's the area? In addition, the speed, viscosity, and viscosity-pressure coefficient need to be defined.

Lubrication Conditions

Boundary Lubrication:
Film thickness less than one times surface roughness; linear bearings

Mixed Lubrication:
Film thickness one to four times surface roughness; ballscrews

(Elasto)Hydrodynamic lubrication:
Film thickness greater than four times surface roughness; ball bearings

Voice your opinion!

To join the conversation, and become an exclusive member of Machine Design, create an account today!