If the distance traveled per unit time is known, the life can be expressed in terms of time, which may be easier to understand. The relationship between the stroke distance, the stroke frequency per minuit, and the life time is expressed by Equation (9)

L

= 2 10 s n1 60 (9)L

L
h : life time(hr) s : stroke distance(m) n1 : stroke frequency per min.(cpm)

Hardness Coefficient (f

):

In a linear system, the guide rail serves the same purpose as an inner race of a ball bearing. Therefore, the hardness of the guide rail plays an important role in determining the rated load. If the surface hardness is less than HRC58, the rated load is reduced. NB uses an advanced heat treatment method to maintain an appropriate level of hardness. However, if guide rails with inadequate hardness must be used, the rated load must be re-calibrated based on the hardness coefficients given in Figure 1-2.

Figure 1-2 Hardness Coefficient
H Hardness Coefficient, f

Temperature Coefficient (f

):

Tracking Surface Hardness, HRC

NB linear systems are heat treated to reduce the amount of wear. Therefore, if the operating temperature exceeds 100 , hardness is reduced and the life of the system is shortened. The variation in hardness with temperature is shown in Figure 1-3.

Figure 1-3 Temperature Coefficient
T

Contact Coefficient (f

):

When two or more linear systems are used in contact with each other, the variation in each system and the accuracy of the mounting surfaces must be taken into consideration. In general, the coefficient values given in Table 1-2 should be used to compute the life.

Temperature Coefficient, f Temperature of Linear System ( )Table 1-2 Contact Coefficient

Applied Load Coefficient (f

number of linear systems in contactand applied to a single shaft contact coefficientf
W

):

When computing the applied load, the weight of the mass, inertial force, moment resulting from the motion, and the variation with time should be accurately estimated. However, it is very difficult to accurately estimate the applied load due to the existence of numerous variables, including the start/stop conditions of the reciprocating motion and of the shock/vibration. Estimation is simplified by using the values given in Table 1-3.

1 1.00 2 0.81 3 0.72 4 0.66 5 0.61 Table 1-3 Applied Load Coefficient operating condition applied load coefficient,f loading condition velocity
W no shock/vibration 15 m/min or less 1.0 1.5 low shock/vibration 60 m/min or less 1.5 2.0 high shock/vibration 60 m/min or more 2.0 3.5

Eng-5