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PROBLEM 8.68
Derive the following formulas relating the load W and the force P exerted on the handle of the jack
discussed in Section 8.6. (a) P (Wr/a) tan (
s
), to raise the load; (b) P (Wr/a) tan (
s
), to lower the
load if the screw is self-locking; (c) P (Wr/a) tan (
s
), to hold the load if the screw is not self-locking.
PROBLEM 8.69
The square-threaded worm gear shown has a mean radius of 2 in. and a
lead of 0.5 in. The large gear is subjected to a constant clockwise couple of
9.6 kip · in. Knowing that the coefficient of static friction between the two
gears is 0.12, determine the couple that must be applied to shaft AB in
order to rotate the large gear counterclockwise. Neglect friction in the
bearings at A, B, and C.
PROBLEM 8.70
In Problem 8.69, determine the couple that must be applied to shaft AB in
order to rotate the large gear clockwise.
PROBLEM 8.69 The square-threaded worm gear shown has a mean
radius of 2 in. and a lead of 0.5 in. The large gear is subjected to a constant
clockwise couple of 9.6 kip in. Knowing that the coefficient of static
friction between the two gears is 0.12, determine the couple that must be
applied to shaft AB in order to rotate the large gear counterclockwise.
Neglect friction in the bearings at A, B, and C.
PROBLEM 8.71
High-strength bolts are used in the construction of many steel structures. For a 1-in.-
nominal-diameter bolt, the required minimum bolt tension is 51 kips. Assuming the
coefficient of friction to be 0.30, determine the required couple that should be applied to
the bolt and nut. The mean diameter of the thread is 0.94 in., and the lead is 0.125 in.
Neglect friction between the nut and washer, and assume the bolt to be square-threaded.
PROBLEM 8.72
The position of the automobile jack shown is controlled by
a screw ABC that is single-threaded at each end (right-
handed thread at A, left-handed thread at C). Each thread
has a pitch of 2.5 mm and a mean diameter of 9 mm. If the
coefficient of static friction is 0.15, determine the
magnitude of the couple
M
that must be applied to raise
the automobile.
PROBLEM 8.72 (Continued)
Copyright © McGraw-Hill Education. Permission required for reproduction or display.
PROBLEM 8.73
For the jack of Prob. 8.72, determine the magnitude of the
couple
M
that must be applied to lower the automobile.
PROBLEM 8.72
The position of the automobile jack shown is controlled by
a screw ABC that is single-threaded at each end (right-
handed thread at A, left-handed thread at C). Each thread
has a pitch of 2.5 mm and a mean diameter of 9 mm. If the
coefficient of static friction is 0.15, determine the
magnitude of the couple
M
that must be applied to raise the
automobile.
PROBLEM 8.73 (Continued)
Copyright © McGraw-Hill Education. Permission required for reproduction or display.
PROBLEM 8.74
The vise shown consists of two members connected by two double-threaded
screws with a mean radius of 0.25 in. and pitch of 0.08 in. The lower member
is threaded at A and B (
s = 0.35), but the upper member is not threaded. It is
desired to apply two equal and opposite forces of 120 lb on the blocks held
between the jaws. (a) What screw should be adjusted first? (b) What is the
maximum couple applied in tightening the second screw?
PROBLEM 8.74 (Continued)
