5.20 NUT AND BOLT TORQUES
The importance of correct torque application cannot be overemphasized. Under-torque can result in unnecessary wear of nuts and
bolts, as well as the parts they secure. Over-torque can cause failure of a bolt or nut from over-stressing the threaded areas.
Uneven or additional loads that are applied to the assembly may result in wear or premature failure. The following are a few simple,
but important procedures, that should be followed to ensure that correct torque is applied.
NOTE: Insure that the torque to be applied is for the size of the bolt shank not the wrench size.
Use the standard torque table provided as a guide for tightening nuts, bolts and screws whenever specific torque values are not
provided in the builders manual. These values are for clean and dry threads. Note that on the smaller bolts the torque is quite low
and is in inch pounds. Do not use a foot pound torque wrench on these bolts, but instead use a torque wrench calibrated in inch
pounds. The propeller manufacturer and the engine manufacturer have specific torque requirements for their equipment. Consult
the appropriate manual for that information. Apply a smooth, even pull when applying torque. When using the 1/4 in. drive
snap-over type torque wrench we recommend practicing with it off the aircraft first until you get the feel of the particular tool's
snap-over feature since it can be rather light depending on the size of the fastener.
Apply the torque to the nut and not the bolt whenever possible. This will minimize rotation of the bolt in the hole and reduce wear.
When the bolt is rotated for final torque the chart values must be modified. When applying torque to a bolt be sure to have a washer
under the bolt head and lubricate the bolt shank. Add to the overall torque value the torque required to overcome the friction
associated with turning the shank of the bolt within the assembly.
When tightening fasteners with self-locking nuts the chart values must be modified. Due to the friction of the locking device
noticeable torque is required just to turn the nut onto the threads and does nothing to actually tighten the parts together and stretch
the bolt (clamp load). This is called friction drag (or prevailing) torque. The friction drag torque must be determined and then added
to the standard torque from the table. Run the nut down to where it nearly contacts the washer or bearing surface and check the
friction drag torque required to turn the nut. (At least one thread should protrude from the nut). Add the friction drag torque to the
standard torque. This sum is referred to as the final (or total) torque, which should register on the indicator or setting for a snap-over
type torque wrench.
As an example illustrating the importance of determining the friction drag torque consider a new AN3 bolt and MS21042-3 all-metal
lock nut. Our tests showed an average friction drag torque of 14 in-lbs (your results may vary). The standard torque for this nut/bolt
combination from the table below is 28 in-lbs. This results in a final torque setting on our wrench of 14 plus 28 or 42 in-lbs. Though
we exceeded the 28 in-lb value listed in the table by using a final torque of 42 in-lbs we are still well within the capability of the nut.
(Incidentally this nut must meet a maximum torque test value of 60 in-lbs per the military standard spec sheet.) Now what if we
completely ignore the friction drag torque and set our wrench to just 28 in-lbs? Recall that it requires about 14 in-lbs (friction drag
torque) just to turn the nut. We subtract 14 from 28 and arrive at only 14 in-lbs of torque(torque being the measurement of friction,
not tension) applied to induce preload (clamp load) in the bolt. Not a satisfactory result.
(Portions of this information has been adapted from AC 43.13-1B Section 7-40.)
AN Bolt
Size
FOOT POUNDS
91.6-125.0
1100-1500
5/8-18
3/8-24
1/2-20
9/16-18
7/16-20
1/4-28
5/16-24
#10-32
13.3-15.8
160-190
480-690
800-1000
450-500
40.0-57.5
66.6-83.3
37.5-41.7
50-70
100-140
INCH POUNDS
20-25
4.2-5.8
8.3-11.6
1.6-2.0
Bolt Size-
Threads Per Inch
Standard Nuts
AN310, AN315, AN365
AN10
AN6
AN8
AN9
AN7
AN4
AN5
AN3
2.3
28
10-32
MS21042-3
7.0
85
1/4-28
MS21042-4
FOOT POUNDS
INCH POUNDS
28
2.3
85
7.0
Self Locking Nut
MS21042-3, MS21042-4
FASTENERS AS PIVOT POINTS
In specific cases, bolts are intended to be used as axis of rotation. The most common example is the attachment of control system
cables. Here, it is intended that the cable end fitting pivot on the bolt. In these instances, the nut must not be torqued to the
standard torque value. Instead a castellated nut, safetied with a cotter pin, is used. Finger-tighten the nut, then install the cotter pin.
The cable end fitting must pivot freely w hen the installation is complete.
MS NUTS
A common rule of thumb for evaluating whether an installed bolt is the proper length has always been that up to three thick washers
are allowed (if more than three are needed the next shorter bolt should be used), and that at least one but no more than three full
threads of the fastener are to be showing beyond the nut. This rule of thumb works for the AN365 nuts that have been commonly
used on RVs for years.
Some of the newer RV kits use the all metal MS21042 self locking nut. The long standing rule of thumb will not work for these nuts
because they are shorter in height than the AN365 nut. For these nuts, modify the rule of thumb to “at least three, but not more
than five threads showing.” This rule of thumb will still allow for meeting the 'maximum use of three washers' rule.
You may have to educate any technical councilors or airworthiness inspectors that perform inspections on your project. Some
inspectors are not familiar with these smaller sized nuts.
PAGE
REVISION:
DATE:
VAN'S AIRCRAFT, INC.
PAGE
REVISION:
DATE:
04/15/13
05-18 RV-ALL
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