Calculation of service life

The service life is defined as the distance that can be traveled before the raceway or rolling elements begin to flake.

Service life can be reduced due to a number of factors including:

- Contamination by foreign substance
- Insufficient lubrication
- Short stroke length or shock forces
- Overloading
- Damage to the plastic end retainers

Technical Definitions:

The dynamic load rating (C) is the constant load, in direction and magnitude, on a linear guide under which the nominal service life is 50km.

The static load rating (C₀) ensures that the deformation of the rail due to a high impact load is no more than 0.0001 times bigger than the diameter of the ball. This is to ensure smooth running.

The allowable static moment (M₀) limits the deformation of rail by ensuring the  ball at the end (most loaded) does not cause a indentation in the rail greater than 0.0001 times the diameter of the ball. Moment loads in the three directioms (M_p, M_r and M_y) are listed for each type of carriage. The allowable static moment M₀ and the static moment load rating M_p can be reviewed by applying a safety factor (f_s).

[Calculation of service life]
Load Calculation:

The forces must be combined into an equivalent load (P_c) in order to calculate the service life. It is important to include drive forces and acceleration torques in this.

The nominal service life is calculated from the dynamic load C and the equivalent dynamic load P_c together with reduction factors listed below.

Where there are several carriages in a system, the service life should calculated based on the carriage subjected to the highest forces.

Formula for calculating the service life:

where:
L = Service Life (km)
C = Basic dynamic load rating (N)
P_c= Calculated load (N)
f_H = Hardness factor*
f_T = Temperature factor*
f_c = Contact factor*
f_W = Load factor*

Reduction Factors: 

Load factor f_W -
If the carriages are subjected to shock or vibration, then the loads seen by the carriages will be higher. Select the load factor from below:
No shock or vibration, f_W = 1.0 to 1.5
Moderate shocks or vibrations, f_W = 1.5 to 2.0
Moderate shocks or vibrations, f_W = 2.0 to 3.0

Temperature factor f_T -
The maximum continuous operating temperature is 80ºC. For this range of temperatures f_T =1. When using special high temperature guides, the temperature factor is available on request.

Hardness factor f_H -
Since the raceways are hardened, f_H =1

Contact factor f_C -
For a classic arrangement of two rails with four carriages, f_c =1
If there is only one rail with two carriages f_c =0.81
Or for one rail with three carriages f_c =0.72

[Calculation of service life]
Lifetime Calculation:

If the length and frequency of the stroke is constant, then the lifetime can be calculated as follows:

where L_h = Life time (hours)
l_s = Length of stroke (mm)
and n₁ = number of strokes per minute (mm^-1)

ACCURACY

The accuracy is determined by the dimension tolerance (and deviation) along with the parallelism of the guide rails.

Dimension Tolerance (mm):

[Accuracy]
Parallelism (µm):

The parallelism stated in the table below is the deviation along the entire length of the rail both vertically (from the top of the carriage [C] relative to the base of the rail [A]) and also horizontally (from the side of the carriage [D] relative to the side of the rail [B]).

[Accuracy]
Preload (µm):

Preload of Linear Motion carriages TWH and TWS (in µm)

Spacer Chain Option (+S)

The carriages can be specified with a spacer chain by adding +S to the end of the part number. The spacer chain holds the balls to prevent them from colliding when they recirculate. The main benefit is a reduction in noise, but it also reduces the friction and enables higher running speeds. The ball chain provides additional pockets for the lubricant and hence it can be relubricated less frequently. A spacer chain should not be used if there is a lot of dirt.