Page 354 - Marks Calculation for Machine Design

P1: Sanjay
January 4, 2005
15:14
Brown.cls
Brown˙C08
APPLICATION TO MACHINES
336
If a factor-of-safety against separation (n separation ) is defined as
P o
n separation = (8.48)
P
Substitute (P o )from Eq. (8.47) in Eq. (8.48) to give
F preload
n separation = (8.49)
P (1 − C)
U.S. Customary SI/Metric
Example 4. Using the joint constant (C) Example 4. Using the joint constant (C)
found in Example 3, determine the load found in Example 3, determine the load
factor (n load ) and the factor-of-safety against factor (n load ) and the factor-of-safety against
separation (n separation ) for a bolted connection separation (n separation ) for a bolted connection
that allows periodic disassembly, where that allows periodic disassembly, where
C = 0.25 C = 0.24
P = 2,500 lb P = 11,000 N
6
3
S proof = 86 kpsi = 86 × 10 lb/in 2 S proof = 600 MPa = 600 × 10 N/m 2
2
2
A T = 0.142 in = 1.42 × 10 −1 in 2 A T = 83.4 mm = 8.43 × 10 −5 m 2
solution solution
Step 1. Use the given proof strength (S proof ) Step 1. Use the given proof strength (S proof )
and tensile-stress area (A T ) in Eq. (8.25) to and tensile-stress area (A T ) in Eq. (8.25) to
determine the proof load (F proof ) determine the proof load (F proof )
F proof = S proof A T F proof = S proof A T
3
6
2
2
2
2
= (86 × 10 lb/in )(1.42 × 10 −1 in ) = (600 × 10 N/m )(8.43 × 10 −5 m )
= 12,200 lb = 50,600 N
Step 2. Use the guidelines in Eq. (8.26) to Step 2. Use the guidelines in Eq. (8.26) to
determine the bolt preload (F preload ) as determine the bolt preload (F preload ) as
F preload = 0.75 F proof F preload = 0.75 F proof
= (0.75)(12,200 lb) = (0.75)(50,600 N)
= 9,150 lb = 37,950 N
Step3. Substitutetheproofload(F proof )found Step3. Substitutetheproofload(F proof )found
in step 1, the bolt preload (F preload ) found in in step 1, the bolt preload (F preload ) found in
step 2, and the given joint constant (C) and step 2, and the given joint constant (C) and
external load (P) in Eq. (8.45) to determine the external load (P) in Eq. (8.45) to determine the
load factor (n load ) as load factor (n load ) as
F proof − F preload F proof − F preload
n load = n load =
CP CP
(12,200 lb) − (9,150 lb) (50,600 N) − (37,950 N)
= =
(0.25)(2,500 lb) (0.24)(11,000 N)
3,050 lb 12,650 N
= = 4.9 ∼ 5 = = 4.8 ∼ 5
=
=
625 lb 2,640 N