It is important to select properly sized components when choosing a Trapezoidal Lead Screw and Nut. The analysis of the product p●Vst will allow you to plot graph No. 1. Three areas are highlighted on the graph, each of which characterises a different degree of suitability for the given task. These regions allow us to make evaluations regarding the appropriate sizing of Lead Screws and Nuts. Proper lubrication is always necessary. Failure to lubricate may cause results to vary significantly.
Graph No. 1 – Sliding Condition for Bronze
Area A : area A is determined by p●Vst = 21 [N/mm2 ● m/min] This area represents optimal working conditions. “Continuous service” is possible in this region, as the amount of heat produced within these limits is low. Nuts which fall within this region will have a long life. Area B : area B is determined by p●Vst = 80 [N/mm2 ● m/min] In this area, mechanisms operate under more strenuous conditions. Regular lubrication is required to contain the erosion of Bronze Nuts and sustain the mechanism’s life. “Continuous service” is possible her but only for a limited period, as the amount of heat produced is such to cause overheating of the nut. The life of a nut is deceased in this zone. Area C : area C is determined by p●Vst = 250 [N/mm2 ● m/min] In this area, a Lead Screw mechanism operates under very heavy conditions. If results fall within this region, “continuous service” is not possible. Even steady lubrication will not prevent high levels of overheating and very fast nut erosion due to heavy fiction between the surfaces in contact.
General Considerations for Bronze Nuts
In all three of the working conditions described, the erosion of bronze nuts can vary greatly depending on the quality and quantity of lubrication used during operation. Therefore, giving exact reference values is difficult when estimating the longevity of a nut. In particular, applications where working temperature is above 1l40/150°C can cause damage to lubricants, and consequently alter the operating conditions and life of a Lead Screw mechanism. In such cases, AccuLeadScrews recommend the use of specialised lubricants designed for use at high temperatures.
Safety Factor for the Forces of Inertia "f i"
The sizing process also requires an analysis of the inertia forces present during acceleration and deceleration. If the value of p●Vst remains within the controlled limits, inertia values are relatively low, and are of an acceptable level. In the presence of varied, irregular motion, the safety factors reported in Chart. No.1 must be considered.
Chart No. 1 - Safety factors with respect to the forces of inertia
Loads with constant, controlled ramps of acc./dec.
from 1 to 0.5
Loads with constant start and stop at tear
from 0.5 to 0.33
Loads and speed greatly variable
from 0.33 to 0.25
Loads in presence of shock and vibrations
from 0.25 to 0.17
Use of the coefficient "f i" is necessary to correct the value returned by the formula “(p●Vst) max", which can be found in graph No. 1. This calculates the maximum permissible sliding speed in relation to the contact surface pressure. The working “area” limits defined in graph No.1 (A, B or C) must also be considered.
The following formula can be used to calculate the p●Vst of a mechanism: