Power Screws & Threaded Fasteners Section IX Power Screws & Threaded Fasteners

Talking Points Power Screws & Threaded Fasteners? Turning Moment and Axial Load Efficiency of a Screw Mechanism Stresses in the Thread

Power Screws & Threaded Fasteners? Power screws: designed to convert rotary motion to linear motion and to exert the necessary force required to move a machine element along a desired path. Provides a means for obtaining large mechanical advantages Such applications as screw jacks, clamps, presses, and air craft control-surface actuators. Occasionally they are used in reverse for such applications as push drills. Threaded fasteners: include through bolts, studs, cap screws, machine screws, set screws, and a variety of more special devices using the screw principle. For threaded fasteners, the ISO metric thread forms has the basic shape and proportions shown in the figure below. The square thread and ISO trapezoidal forms are commonly used for power screws, as shown in the following figure.

Power Screws & Threaded Fasteners? – Cont. Definitions Pitch: is the distance from a point on one thread to the corresponding point on the next adjacent thread, measured parallel to the axis. Lead: is the distance the screw would advance relative to the nut in one rotation. For a single-thread screw, lead is equal to pitch. For a double-thread screw, lead is equal to twice the pitch. Helix Angle a: is related to the mean radius rm by the equation: qn is the slope angle of the thread profile in the normal section: where cosqn appears in the equations to follow, it is frequently replaced by cosq. This yields an approximate equation but, for the usual small values of a, introduces no great error. The major diameter (do) is the largest diameter of a screw thread. The minor diameter (di) is the smallest diameter of a screw thread.

Turning Moment and Axial Load The torque required to raise a load W is given by: Where: The torque required to lower the load W (in the direction of the load) is given by: This torque may be either positive or negative. If positive, work must be done to advance the screw. If negative, means that the axial load alone will cause rotation. In this the screw is said to be overhauling.

Efficiency of a Screw Mechanism It is the ratio of the work output to work input.

Stresses in the Thread It can be estimated by considering the thread to be a short cantilever beam projecting from the root cylinder. The beam load is taken to be the axial screw load W, presumed concentrated at the beam radius, i.e. at one-half the thread depth h. The bending stress at the root of the thread is, very nearly: b = is the width of thread section at the root; n = number of threads. The mean transverse shear stress is: Bearing pressure: This pressure between the surface of screw and nut threads may be a critical factor in design, especially for power screws. It is given approximately by:

Stresses in the Thread – Con. Stress in the root cylinder of a screw: It may be established by considering loads and torques carried by the bare cylinder as follows: 1) The torsional shear stress: 2) The direct stress:, which may be either tensile or compressive, is: ri = is the root radius of the screw.