PS Magazine - TB 43-PS-572
PS, The Preventative Maintenance Monthly
ISSUE 572
JULY 2000
PS 572
59
3. Loosen with
2nd wrench
4. Retorque to spec
1. Finger tighten
2. Torque to
specified value
Use the Right Washer
Always use the washer called for in your manuals, too. It must be able to stand
up to the torque without thinning out. A soft washer will lose its shape under
load, leaving the bolt loose enough to fail.
Once Is Enough
You won’t get the same clamping force with the same amount of torque on a
nut or bolt that’s been used a number of times.
Increased friction because of deformed threads takes more and more of the
torquing effort, meaning you get less and less holding force.
As an example, a new bolt might have a tension load of 13,250 pounds with
170 lb-ft of torque. The sixth time the nut is installed, the 170 lb-ft of torque
produces only 7,500 pounds of load, a loss of 43.5 percent.
If your TM calls for new fasteners, use them. Otherwise, you’re asking for
trouble.
After making sure
the threads of the nut
and bolt are clean, do the following:
1.
Run the nut up by hand until it
begins to tighten.
2.
Torque the nut to the specified
value in one steady, continuous mo-
tion.
3.
Loosen the fastener with another
wrench. (Never use a torque wrench
to loosen a fastener.) Loosening cleans
the threads of burrs and dirt and lets
the mating surfaces seat and align with
each other.
4.
Torque to specs.
Here’s a
prescription
for proper
torquing.
..
If you torque a bolt beyond its built-in limits (and it doesn’t break), it stretches
so far it can’t snap back. You won’t know it’s sprung, so you keep on torquing.
Even if you get the torque reading you’re looking for, it’s wrong. The first bit
of stress, and the bolt will snap or the nut will back off.
It’s a sure bet what you’ve joined together will come apart sooner or later.
Wet or Dry Torque?
Some 90 percent of the torque you apply goes to overcome friction. Only 10
percent goes to tighten.
When you use a lube, you reduce friction. That means the same amount of
torque will create more tightening force, probably too much.
On the other hand, using a wet value on a dry bolt will not get the right amount
as you can
see, if you applied
grade 8 torque
to a
grade 5 bolt
, you'd
destroy
it.
Torquing Tips
PS 572
58
JUL 00
The right torque depends on what the bolt is made of and how it’s made.
of clamping force.
If you use lube, make sure you use the wet torque value in your TM, not the
dry one.
The first bolt of each diameter is
Unified Coarse.
..
... the second is Unified Fine.
Use this chart only if your TM gives
no torque values.
DIAMETER/
TORQUE LB-FT TORQUE LB-FT TORQUE LB-FT
THREADS
NO DASHES
3 DASHES
6 DASHES
PER INCH
(GRADE 2)
(GRADE 5)
(GRADE 8)
1
/
4
-20
3—5
6—8
10—12
1
/
4
-28
4—6
8—10
9—14
5
/
16
-18
7—11
13—17
19—24
5
/
16
-24
7—11
14—19
23—28
3
/
8
-16
14—18
26—31
39—44
3
/
8
-24
15—19
30—35
46—51
7
/
16
-14
23—28
44—49
65—70
7
/
16
-20
23—28
44—54
69—79
1
/
2
-13
32—37
65—75
95—105
1
/
2
-20
34—41
73—83
113—123
9
/
16
-12
46—56
100—110
145—155
9
/
16
-18
47—57
107—117
165—175
5
/
8
-11
62—72
140—150
200—210
5
/
8
-18
67—77
153—163
235—245
3
/
4
-10
106—116
260—270
365—375
3
/
4
-16
115—125
268—278
417—427
7
/
8
-9
165—175
385—395
595—605
7
/
8
-14
178—188
424—434
663—673
1-8
251—261
580—590
900—910
1-14
255—265
585—634
943—993
1
1
/
4
-7
451—461
1070—1120
1767—1817
1
1
/
4
-12
488—498
1211—1261
1963—2013
1
1
/
2
-6
727—737
1899—1949
3111—3161
1
1
/
2
-12
816—826
2144—2194
3506—3556
Typical Torque Values
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