Linear speed is dictated by the functional requirements of the application. Rotational speed is a function of the linear speed and the lead of the screw. Rotational speed (rpm) is equal to the linear speed (in./minute) divided by the lead of the screw (in./rev.). Leads are listed in the Screw/Nut Engineering Data for each screw series and size. For example, a 1 1/2 x .500 Hi-lead® screw is to move a load at 100 in./min. This will require a rotational input speed of 200 rpm (100 ipm/.500 ipr = 200 rpm).
Speed - Power Screws
Acme screws are most commonly used at 100 rpm or less, with some applications running in the 300 rpm range. Because of their relatively low efficiency, when faster traverse rates are needed, Hi-leads®, Torqsplines® or Ballscrews should be considered. Bear in mind that for an Acme screw drive system with an efficiency of 30%, the remainder of input energy (70%) ends up as heat. Heavily loaded and fast Acme drives heat up very quickly and may need very short duty cycles to prevent seizure. Hi-leads®, Torqsplines® and Ballscrew drives are much better suited to high traverse rates. Their mechanical efficiencies are higher resulting in much less heat generation.
Speed - Ballscrews
Ball velocity in a Ballscrew should not exceed 3,000 rpm x in. (rotational speed (rpm) times the nominal diameter (in.)). For example, a 3/4 x .200 size Ballscrew should be limited to 4,000 rpm (3,000/.750 = 4,000 rpm). For applications requiring speeds beyond 3,000 rpm x in., use a larger lead, a larger diameter, or contact Roton Application Engineering. Freewheeling Ballscrews work best at 350 rpm and less. This limit is imposed by the dynamic action of the stop pins contacting the ball retainer. For operation of Freewheeling Ballscrews beyond 350 rpm, contact Roton Application Engineering. In addition to the above guidelines, each screw drive system should be evaluated for safe rotational speed so that natural frequency vibrations are avoided (see Critical Speed section below).
Screw end fixity is the engineering term for screw end support. Fixity is an important element in screw and nut drive systems. The rigidity of the screw end support determines the screw drive system’s resistance to column buckling and limit of speed of rotation to avoid natural frequency vibration. (See Column Loading and Critical Speed sections.) Theoretically, there are only 3 types of screw end mountings – free, supported and fixed. Free is just that – no support of any kind. The illustrations in Table 34 demonstrate “fixed” and “supported” screw end fixities. A supported end will resist axial and radial loads but not moment (overturning) loads. A fixed end will support axial, radial and moment loads.
Types of End Fixity
Fixed ends offer the highest column load support and the highest resistance to vibration. A supported end and a free end should never be used. The relative rigidity and the factors for critical speed and column loading are listed in Table 40. These factors show the relative effect of end configuration on a screw system’s ability to support column loads and its vibratory limit of critical speed. For more detail on how these factors are used, see Column Loading, Critical Speed and the Useful Formulas sections.
Types of End Fixity
Critical speed is the engineering term for the first natural frequency of vibration of a rotating shaft. Whether mounted horizontally or vertically, a rotating screw system must be operated below its critical speed to avoid vibration, noise and possible failure. Critical speeds are shown in graphic form in Figure 27. Using the minor diameter of the screw from the Screw/Nut Engineering Data section for the selected screw and unsupported length of the screw, find the critical speed in rpm from the graph. Using the formula for critical speed, the safe operating speeds can be calculated. If your desired rpm is greater than the safe speed, increase the screw diameter, increase the screw lead (and decrease the rpm) or change the end fixity to provide more stiffness.
For example, a 1 x .333 Ballscrew is selected to run at 200 in. per minute linear speed with a 70 in. span. The screw will have one end fixed and one end supported and a factor of safety of 2.0 will be used.
Using the screw minor diameter for a 1 x .333 Ballscrew of .75 in. from Table 26, the critical speed can be calculated from the formula or determined from the graph (Figure 28). Reading the graph for a minor diameter of .75 and a span length of 70 in., the critical speed is approximately 720 rpm. The safe operating speed is 558 rpm (720 x 1.55/2.0) where 1.55 is the correction factor for one end fixed and one end supported and 2 is the factor of safety. The rpm required for 200 in. per minute linear speed is 600 rpm (200 ipm/.333 in./rev.) where .333 is the screw lead. Since 600 rpm is greater than the safe operating speed of 558 rpm, a screw with a larger lead or diameter must be selected. Using the same desired conditions as above, a 1 x .500 Ballscrew with a lead of .500 in./rev. will require only 400 rpm (200 ipm/.500 in./rev. = 400 rpm). Since 400 rpm is below the safe operating speed of 558 rpm, a 1 x .500 Ballscrew will provide the desired linear speed at a safe operating rpm. For data points beyond the range of the graphs, use the formula for Safe Operating Speed in the Useful Formulas section.