FRICTIONAL FORCES ON SCREWS In-Class Activities: Check Homework, if any Reading Quiz Applications Analysis of Impending motion Analysis of a self locking screw Concept Quiz Group Problem Solving Attention Quiz Today’s Objectives: Students will be able to: a) Determine the forces on a square- threaded screw.

READING QUIZ 1. A screw allows a ______ moment M to lift a _________ weight W. A) (large, large)B) (small, small) C) (small, large)D) (large, small) 2. A screw is self locking if it remains in place under __________ loads. A) any axial B) small axial C) any rotational D) small rotational W

APPLICATIONS How can we determine the force required to turn a screw? Some screws are self locking, meaning it remains in place under any axial load. How do we determine if this is the case? Screws are sometimes used not as fasteners, but as mechanisms for transmitting power from one part of a machine to another.

How much friction is needed to create a self locking apparatus? APPLICATIONS (continued) The design of a turnbuckle requires knowledge of self locking properties and the minimum moment M required to turn the machine.

ANALYSIS OF A SCREW W A screw is a simple machine in which a small Moment M is used to lift a large weight W. To determine the force required to turn the screw, it is necessary to draw an FBD of the screw thread.

ANALYSIS OF A SCREW (continued)

EXAMPLE

EXAMPLE (continued)  F X = -N sin(Θ) + M/r –.25 N cos(Θ) = 0  F Y = N cos(Θ) –.25 N sin(Θ) - W = 0 Solving the above two equations, we get M = 6.37 N * m M/r W F N

GROUP PROBLEM SOLVING