Plastic Trapezoidal Lead Screw Nuts have many advantages, but there are also a number of considerations to take into account during the selection process.
Correction Factor for Working Environment Temperature
Using nylon plastic nuts, the value of p●Vst must be adjusted to account for the temperature of the working environment. Plastic softens at higher temperatures, and becomes less able to withstand heavy loads. At lower temperatures, plastics become harder, and are able to handle heavier loads. The correction factor "f t" can be seen in graph No. 4.
Correction Factor Dependent on Intermittent Use
Plastic nuts can be operated intermittently for relatively short spaces of time to avoid reaching high operating temperatures, and to avoid losses in efficiency.
Temperature is an additional factor which contributes to the values showing working condition in graph No. 2, which shows nuts in continuous operation. The working conditions of a nut can be improved when the amount of time operating in intermittent cycles is higher than the time spent in continuous cycles. The values of factor “f c” and the curves of “x” within graph No. 5 can be used to explore the relationship between downtime and working time of a nut, and the impact which this has upon the working conditions of a mechanism.
- 2 x represents a downtime double the length of the working time.
- 3 x represents a downtime triple the length of the working time.
- 4 x represents a downtime quadruple the length of the working time. To use this graph, find the value for working time on the horizontal axis, then use the intersecting line of the chose length of work time in relation to down time (2x, 3x, 4x, etc.), then move horizontally to find the value "f c".
The values found in the earlier chart and graphs, "fi", "ft", "fc", can be used to correct the value of "(p●Vst)” to find the optimal working conditions of a nut.
To calculate the maximum p●Vst of a particular mechanism, use the formula:
p ● Vst am = (p ● Vst) max ● fi ● ft ● fc
Lifetime of the Plastic Nut
Using the values returned from these formulas, it is possible to estimate the lifetime a plastic nut may have. The parameters that can have an impact upon the life of plastic nuts are:
- Contact surface pressure p [N/mm2]
- Sliding speed Vst [m/min]
- Resistance to the erosion of the nut’s material, which can be derived from
- Correction factor of fc, taken from graph No.5
Data shown below is only applicable for coupling of plastic nuts with our precision rolled Lead Screws, as we can guarantee a surface roughness of less than 1 μm Ra.
The use of Plastic Lead Screw Nuts with lathed screws is not recommended.
The following calculations are intended for screws working at temperatures of approximately 20/25°C with a relative humidity of between 30% and 70%. For working environment outside of these temperature and humidity boundaries, please consult our technical advice team.
To calculate the lifetime of a plastic nut, use the following formula:
m = increase in axial play between the nut and the screw in respect of the initial value [mm]
fc = correction factor taken from graph No.5
p = contact surface pressure (see general sizing criteria) [N/mm2]
Vst = sliding speed (see general sizing criteria) [m/min]
k = constant of resistance to wear
Value of the constant k for nylon plastic nuts: k = 10,5 ● 10-5