CH-8 LEC 34 Slide 1

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Chapter 8
Screws, Fasteners, and the Design of Nonpermanent
Joints
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Chapter Outline
8-1 Thread Standards and Definitions8-2 The
Mechanics of Power Screws8-3 Strength
Constraints 8-4 Joints-Fasteners
Stiffness 8-5 Joints-Member Stiffness 8-6 Bolt
Strength 8-7 Tension Joints-The External
Load8-8 Relating Bolt Torque to Bolt
Tension 8-9 Statically Loaded Tension Joint with
Preload 8-10 Gasketed Joints 8-11 Fatigue Loading
of Tension Joints 8-12 Shear Joints 8-13 Setscrew
s 8-14 Keys and Pins 8-15 Stochastic
Considerations
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Announcements

• HW 5 Ch. 18, on WebCT
• Due Date for HW 5 is Mon. DEC. 31, 2007
• Quiz on Ch. 18, Mon. DEC. 31, 2007 ?????

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LECTURE 34
8-2 The Mechanics of Power Screws
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Example-1
A power screw is 23 mm in diameter and has a
thread pitch of 7 mm.(a) Find the thread depth,
the thread width, the mean and root diameters,
and the lead, provided square threads are
used.(b) Repeat part (a) for Acme threads.
GivenDiameter of the power screw, d 23
mmThread pitch, p 7 mm
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The Mechanics of Power Screws
A power screw is a device used in machinery to
change the angular motion into linear motion, and
usually, to transmit power.

• Applications
• Lead screws of lathes
• Screws for vises, presses and jacks

Figure 8-4 The Joyce worm-gear screw jack.
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The Mechanics of Power Screws
In Figure 8-5 a square threaded power screw with
single thread having a mean diameter dm, a pitch
angle p, and a lead angle ?, and a helix angle ?
is loaded by the axial compressive force F.

• We wish to find an expression for the torque
  required to raise this load, and another
  expression for the torque required to lower the
  load.

Figure 8-5 Portion of a power screw (Square)
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Figure 8-6 Force Diagrams (a) Lifting the load
(b)lowering the load
Imagine that a single thread of the screw is
enrolled or developed (Fig. 8-6) for exactly a
single turn. Then on edge of the thread will form
the hypotenuse of a right triangle whose base is
the circumference of the mean-thread- circle and
whose height is the lead. The angle ? is the
lead angle of the thread . For raising the load a
force PR acts to the right and to lower the load,
PL acts to the left.
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(a)
For raising the load
(b)
For lowering the load
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Eliminating N from the previous equations and
solving for P gives
(c)
For raising the load
(d)
For lowering the load
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Next, divide the numerator and the denominator of
these equations by cos ? and use the relation
(e)
For raising the load
(f)
For lowering the load
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The torque is the product of the force P and the
mean radius
(8-1)
(8-2)
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Self Locking Condition
(8-3)
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Self Locking Condition

• The critical coefficient of friction for the lead
  concerned,
• If f fcr the nut is on the point of moving
  down the thread without any torque applied.
• If f gt fcr then the thread is self-locking in
  that the nut cannot undo by itself, it needs to
  be unscrewed by a definite negative torque
  Clearly self-locking behavior is essential for
  threaded fasteners.
• Car lifting jacks would not be of much use if the
  load fell as soon as the operating handle was
  released.

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Power Screw-Overhauling
If f lt fcr then the thread is  overhauling in
that the nut will unscrew by itself under the
action of the load unless prevented by a positive
tightening torque.

• Some applications of power screws require
  overhauling behavior.
• The Archimedean drill
• 2. Pump action screwdrivers
• (Yankee screw drivers)
• These devices incorporate very
• large lead angles

Increasing lead (angle) ? overhauling
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Power Screw-Overhauling
Sensitive linear actuators may incorporate
recirculating ball screws such as that
illustrated here to reduce thread friction to
levels which go hand-in-hand with overhauling.
decreasing thread friction ? overhauling
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Power Screw-Overhauling
Sensitive linear actuators may incorporate
recirculating ball screws such as that
illustrated here to reduce thread friction to
levels which go hand-in-hand with overhauling.
decreasing thread friction ? overhauling
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Efficiency

• If we let in Eq. (8-1), we obtain

(g)
which, is the torque required to raise the load.

• The efficiency is therefore

(8-4)
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Efficiency
f
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Power Screw- ACME Thread
F is parallel to screw axis i.e. makes angle a
14.5 with thread surface ignoring the small
effect of l, the resultant normal force N is
F/cos a . The frictional force f N is increased
and thus friction terms in Eq. (8.1) are modified
accordingly
Torque required to raise load F
(8-5)
ACME thread is not as efficient as square thread
because of additional friction due to wedging
action but it is often preferred because it is
easier to machine.
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Power Screw with Collar
In most of power screw applications (load
lifting) a collar is to be designed. The presence
of collar increases the friction torque. A thrust
collar bearing must be employed between the
rotating and stationary members in order to carry
the axial component
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Power Screw with Collar
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Power Screw with Collar
If is the coefficient of collar friction,
the torque required is
fc collar friction coefficient dc collar mean
diameter
(8-6)
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Power Screws-friction coefficients

• Friction wears thread surface for safe
  applications Max thread bearing pressure is given
  in Table 8-4.

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Power Screws-friction coefficients
Table 8-5 Coefficients of friction f for
Threaded Pairs
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Power Screws-friction coefficients
Table 8-6 Thrust Collar friction coefficient, fc

• Coefficients of friction around 0.1 to 0.2 may be
  expected for common materials under conditions of
  ordinary service and lubrication.

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Example-2

• Problem 8.8 (modified)
• Given
• 5/8-6ACME? i.e. d5/8 and N6
• ffc 0.15
• dc7/16 in
• P 6 lb
• Larm2 3/4 in
• Required
• F, efficiency, Self-Lock?

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Example-2 (Cont.d)
Lever torque
p/2 1/2N
d
l 1/N
R
Clamping force
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Example-2 (Cont.d)
Efficiency
Self-lock which is clear that it is self
lock