The primary concern in any screw and nut system is the amount of load to be moved. The load must be determined by the designer before the proper type and size of screw can be selected. If the load is to be moved vertically, the nut load is equal to the weight to be moved plus the friction load. If a load is to be moved horizontally, the weight must be multiplied by the coefficient of friction to determine the net load on the nut. Loads can also be measured with a spring scale or dynamometer. For loads which are neither horizontal or vertical, this may be the easiest method. (Refer to Table 39 for Typical Coefficients of Friction.) The higher the load, the larger the size of the screw and nut that will be needed. Using the Screw/Nut Engineering Data provided for each screw series, select a size with static and operating load ratings as large or larger than the application loading.
Types of Loading
Tension loading is always preferred in any screw and nut system. In compression loading, screws may fail by elastic instability (buckling) and safe column loading must be investigated. (See Column Loading, and Torsional & Axial Deflection sections for more comments on the effects of load on screw and nut systems.) All screw and nut systems perform best when loaded on their axes. This is called axial loading because the screw and nut are loaded in line with the central axis. Radial (side) loading and off center (moment) loads are detrimental and should be avoided or minimized.
Using the drive torque ratios from the Screw/Nut Engineering Data for the selected screw series, the torque can be easily calculated. The drive torque is equal to the load (lbs.) times the drive torque ratio (in.-lbs./lbs.). For example, the torque required to drive 1,000 lbs. using a 1 – 5 size Acme screw is 102 in. – lbs. (1,000 lbs. x .102 in. – lbs./lbs. = 102 in.-lbs.). The drive torque ratios listed are for the screw and nut only. Support bearings and other drive components will require additional torque. Also, additional torque may be needed for acceleration and to overcome starting friction in the drive system which is often higher than running friction. When sizing motors and other drive components, this additional torque should be included.
Efficiency and Backdriving
The mechanical efficiency of screw drive systems is often confusing. This is because unlike most power transmission components, V-belts, sheaves, timing belts, chain drives, and gear systems (with the notable exception of worm gear systems), screw drive systems actually exhibit two efficiencies – one in the drive direction (torque to thrust conversion) and one in the backdrive direction (thrust to torque conversion). The Efficiencies vs. Lead Angle graph (Figure 26) shows the two efficiency curves for a continuum of general screws against a mating Bronze nut (coefficient of friction .15). For a screw at 8° lead angle, the forward efficiency is 45% and the backdrive efficiency is -13%. The negative backdrive efficiency means that the screw is self-locking, that is, some drive torque is required to lower a load. Assuming a lead of 1 in./rev. and a load of 1,000 lbs. the forward drive and backdrive torques can be calculated by using the efficiencies from the graph and the equations for drive torque shown in the Useful Formulas section. Note that the backdrive torque value is negative when the backdrive efficiency is negative. This indicates that the screw is self-locking and that torque in a direction opposite from the drive direction is needed to lower the load. Again with reference to the Efficiencies vs. Lead Angle graph (Figure 26), as the lead angle increases, a screw with a lead angle of 20° has a forward efficiency of 65% and a backdrive efficiency of 52%. The backdrive efficiency is now greater than zero indicating that the screw is not self-locking and braking torque will be needed to sustain a load. Looking at it another way, the backdrive torque calculated represents the torque produced on the screw by a linear force on the nut. Hilead(r) and Torqspline(r) screws use the principle that increasing lead angles dramatically increases the efficiency of power screws. The Efficiencies vs. Lead Angle graph shows efficiencies for the three power screw series – Acmes, Hileads(r) and Torqsplines(r). Note that efficiency rises steadily as the lead angle increases. Ballscrews, which use rolling friction in place of the sliding friction of the other screw series, exhibit efficiencies that do not change with lead angle. The Efficiencies vs. Lead Angle graph shows Ballscrew forward and backdrive efficiency at constant values of 90% forward efficiency and 80% backdrive efficiency for the entire range of lead angles. In actual laboratory measurements, low lead angle Ballscrews do show slightly more efficiency than larger lead angle Ballscrews, but not enough to be significant for commercial Ballscrew applications. No problems should be encountered by assuming 90% forward drive and 80% backdrive efficiency for the entire range of Ballscrew leads and lead angles.