Date: Wed, 6 Aug 2003 13:25:27 -0600
Reply-To: don spence <dspence@OANET.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: don spence <dspence@OANET.COM>
Subject: Re: Torque wrench usage
In-Reply-To: <200308061740.h76HeQ901284@mail.oanet.com>
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John's comments re lubricating (or not) nuts and studs before torquing
raised a question or two so off to the web.
On Wednesday, August 6, 2003, at 10:53 AM, John Rodgers wrote:
> A a matter of good
> mechanical practice, the the torque values on bolts and nuts are
> established with no lubricants, and when a lubricant is added, it
> changes the parameters of the established torque for the part. So
> lubricants SHOULD NOT be added except where specified by the
> manufacturer.
WEB search results:
1: For greatest accuracy, use a thread preparation, either grease,
lube, or an appropriate grade of Loctite, on the threads of the bolts
you wish to tighten.
2: When using a torque wrench there are some basic preparatory steps to
consider. First, make sure the threads of the fastener and the mating
hole are clean and free of rust, dirt, carbon and goop. Debris in the
hole or on the threads will cause galling of the threads and give an
inaccurate torque reading. Secondly, if no special sealers or
instructions are given for torque applications are given, use a small
dab of high temperature grease on the threads and where the fastener
head contacts the part. Use of this lubricant will keep the fastener
from sticking and reduce the possibility of galling and stripping. In
addition the grease will assure the proper tension of the threads to
the mating parts when the correct torque is applied. It will also make
the fastener easier to remove in the future.
Now you ask; why should I use an accurate torque wrench? Well lets
talk about torque and its properties. You see, nuts and bolts are held
together by tension or elasticity. Some bolts are measured via stretch
but we will discuss that issue some other time. Threads provide a
continuously inclined plane, which creates a wedge for the bolt when
you tighten the nut. Like a rubber band, metal is elastic, and will
stretch a little as you tighten a nut. If you over tighten a nut you
will go past the limits of the metal's elasticity and strip the
threads. If you under tighten, the bolt will not stretch and thus will
loosen more easily through normal use. A torque wrench allows you to
tighten the bolt and nut to the correct tension, so it will stay in
place without loosening.
3: Torque is simply a twisting or turning motion around the axis of the
nut or bolt. Torque wrenches measure resistance to rotation. This
resistance can be correlated to, but is not a direct measurement of,
fastener tension. Generally, the higher the resistance to rotation, the
greater the tension on the threaded fastener.
Typically, some type of lubrication is desired in the thread and at the
bolt head. Without lubrication, a higher twisting resistance is felt by
the torque wrench. The torque wrench reads only this resistance to
rotation, it does not directly read the tension in the bolt. Be sure to
check manufacturers suggestion on lubrication; many manufacturers
require a certain weight of oil or other liquid.
Another option for thread preparation is a thread locking compound,
such as from Locktite®, Wurth®, and Devcon®. This is a fluid that is
applied to clean threads. The fluid provides some lubrication during
torque. When sealed from air, the fluid hardens and expands as it
cures. This provides resistance to loosening and also forms a seal to
water and corrosion. Thread locking compounds can be useful when parts
cannot be fully torqued. Thread lockers should not be used to replace
proper torque and thread pre-load when clamping load is important.
Generally, the service removable grades are used, such as Locktite®
#222 or #242.
Cross threaded parts will cause more resistance than correctly aligned
threads. A torque wrench is unable to distinguish this problem. Don't
do something stupid and torque down a cross-threaded head bolt! If you
do, we're transferring you to a Ford group.
It is important to understand torque unit specifications. Be sure you
read the manufacturers suggested torque requirements; remember, 125
in/lbs is NOT the same as 125Ft/lbs! Don't screw up and break
something.
The most common reason for threaded fasteners loosening is simply that
they were inadequately secured during initial assembly. Vibration,
stress, or use/abuse usually will not cause a properly sized torqued
fastener to loosen.
Now you ask; why should I use an accurate torque wrench? Well lets
talk about torque and its properties. You see, nuts and bolts are held
together by tension or elasticity. Some bolts are measured via stretch
but we will discuss that issue some other time. Threads provide a
continuously inclined plane, which creates a wedge for the bolt when
you tighten the nut. Like a rubber band, metal is elastic, and will
stretch a little as you tighten a nut. If you over tighten a nut you
will go past the limits of the metal's elasticity and strip the
threads. If you under tighten, the bolt will not stretch and thus will
loosen more easily through normal use. A torque wrench allows you to
tighten the bolt and nut to the correct tension, so it will stay in
place without loosening.