Bearing Design Considerations

Application Variables

To determine the best linear bearing product or system for your application it is necessary to know:

  • Amount of load
  • How the load is applied,/li>
  • Length of stroke

Coefficient of Friction

Nook linear bearings exhibit an extremely low coefficient of friction ranging from 0.0008 to 0.0035. Coefficients of static and rolling friction are used to estimate the force required to overcome frictional resistance. The formulas for determining static and rolling frictional resistance are:

Static Friction:

Fs = L x fs

Rolling Friction:

Fd = L x fd
Where:
Fs = Static frictional resistance (lbs)
Fd = Dynamic frictional resistance (lbs)
L = Applied radial load (pounds)
fs = Coefficient of static friction
fd = Coefficient of rolling friction

The tables show the coefficients of friction for Nook Linear Bearings operating on hardened and ground shafts of recommended diameters.

COEFFICIENTS OF STATIC FRICTION (Fs)
Type of Bearing Lubrication Load In % Of Rated Load
  100% 75% 50% 25%
Any .0024 .0026 .0029 .0035

COEFFICIENTS OF ROLLING FRICTION (Fd)
Type of Bearing Lubrication Load In % Of Rated Load
  100% 75% 50% 25%
None .008 .009 .0013 .0018
Oil .0012 .0013 .0016 .0021
Grease .0013 .0015 .0019 .0026

There are other variables that affect the dynamic frictional resistance of linear bearings. These variables include:

Lubrication:

Dry linear bearings exhibit the lowest coefficient of friction. Friction values for lubricated bearings are higher due to the presence of lubricant surface tension.

Seals:

Non-linear seal drag occurs because of the geometry and the materials used in the bearing seals.

Contamination:

Foreign particles restrict free rolling of the bearing balls and will contribute to an increase in dynamic frictional resistance.

Lubrication

A lubricant formulated for rolling friction should be used with Nook Linear Bearings. In applications where operating speeds are low and loads are light, Nook linear bearings can be used without lubrication at a reduced life. However, to protect the highly polished bearing surfaces from corrosion and wear, a lubricant is recommended.

Where linear speeds are high, a light oil should be used and provision for re-lubrication should be made to avoid operating the bearings dry. For typical applications, a medium to heavy oil has good surface adhesion and affords greater bearing protection. Linear Bearings 2" diameter and above may use high pressure lithium grease such as Shell Alvania #2 for moderate speed applications. Lubricants containing additives such as molydisulfide or graphite should not be used.

Nook Linear Lube LBL-1 liquid is a good, all purpose lubricant for use with linear bearings. See page 198 for more information.

Maximum and Normal Load Ratings

The required design life, the shaft hardness, and a bearing dynamic load rating affect the load that can be applied to a Nook linear bearing. Two dynamic load ratings are given for each bearing size based on the rotational orientation of the bearing.

Normal Load Rating

The Normal Load Rating is used in applications where the orientation of the ball tracks relative to the load cannot be controlled. The Normal load rating is based on a load imposed directly over a single ball track. The Normal load rating shown in the specification tables is slightly greater than would be mathematically calculated based on one track loading because it assumes that the load is shared to some degree by one or more of the adjacent ball tracks.

Maximum Load Rating

The Maximum load rating assumes that the load is applied midway between two ball tracks as illustrated below. In this orientation the load is distributed over the maximum number of bearing balls (See Image 1 in Figure 1 Below for diagram.)

Load Life Determination

The Normal and Maximum load ratings are based on a Rc 60 shaft hardness and a travel life of two million inches. For linear bearing system operating at less than full rated load, the Load-Life Curve may be used to determine the travel life expectancy (See Image 2 in Figure 1 Below for diagrams.)

Shaft Hardness

If shafting other than standard alloy Nook HG shafting is used, the Shaft Hardness Curve establishes a shaft hardness correction factor, Rh. When calculating the equivalent load, this factor compensates for the effect of hardness (See Image 2 in Figure 1 Below for diagrams.)

Equivalent Load

An equivalent load value can be calculated when sizing linear bearings for applications at conditions other than maximum rating.

Equivalent Load Formula:

Le = La / (RL x Rh)
Where
Le = Dynamic Equivalent Load (The minimum bearing capacity to meet design life requirements)
La = Applied Load (Actual Load)
RL = Load Life Ratio Factor (from chart)
Rh = Shaft Hardness Ratio Factor (from chart)

Bearing Installation

In most installations, Nook linear ball bearings are designed to slip-fit into the housing bore and secured by one of the following means:

Between an internal housing shoulder and a threaded cap.
Between external retaining rings
Between internal snap rings in the bore of the housing.

The bore diameter required to maintain recommended bearing/ shaft clearance is given in the EXCEL™ linear bearing information section. The bore does not affect clearance between an LBB bearing and a shaft (See Image 3 in Figure 1 Below for diagrams.)

Figure 1