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A three stage torque is needed to evenly load the head and gasket.
A torque + torque + angle is not just for stretch bolts. And you need to also understand the compressabiltiy of the subject head gasket. Longer bolts are also more elastic as well as alloy heads.
For a non stretch bolt you torque to an intermediate level. For those lower levels of torque, the unknown of the friction is smaller. Then a fixed angle on top of that creates the elastic stress load in the bolt without the torque level being so effected by friction.
Your socket head cap screws will have a lot of friction at the head bolt washer. The small area of the socket head screw will create this. If the washer is hardened and ground that will help. I would use two 3mm washers which might be commercially available. The two washers might turn on each other quite freely.
Arp uses the torque method and has a special moly lube to reduce friction.
Full threaded bolts are softer than part threaded, more elastic and stretchy. So a change in this detail will change the result. Arp's intent is to torque to 75% of the bolts strength.
Arp stated that their torque values can be/should be reduced 'a bit' for alloy heads, but they were obviously not going to give any application specific advice. The said to follow Saabs specs. I stated that there is no Saab spec for a non-stretch bolt.
From what I have read, stretch bolts are good for torque machines that do not have a feel for whats happening. So the stretch creates a stable end point stress. Not the best result, but common practice these days.
As friction is such a variable, and as the threads change their surface when used, IE black oxide wears off etc, they want you to take each fastener to full torque 5 times and backing off, then retorque. The have two torque specs, one for 30W oil and the other for their moly lube.
In some industial work, a stud in inserted, the nut and washer are run down, a puller cap is threaded on, a hydraulic machine pulls on the stud to achieve target stress, the nut is then snugged down, hydraulic released to allow nut and washer and thread roughnesses to be settled, hydraulic pressure applied again, nut released and re-tightened. Another variation is to use a stud with a hole drilled through its stretch length. A guage with an indicator measures amount of stretch in the stud as things are tighened. Can also be done in a drilled bolt. Very expensive and can be used in a study to determine standard procedures. Note that the hydraulic pull method does not leave the fastener with a tortional load. For rod cap bolts where the head and end of the bolt are exposed, one can directly measure bolt stress. But many fasteners are 'blind' and direct measurement is not possible. One can grid the end of a bold to a true flat surface, and also the head of the bolt. Actual lenght of the double end ground bolt is not critical. An ultrasonic probe can then detect the bolt stretch. Better than drilling the fastener I expect. Bench studies can be done to determine if highly stressed steel changes the speed of sound propogation in the steel. Prior work and practice may dictate that such corrections are not significant considering variations in bolt to bolt strength cosidering that there is perhaps a 25% safety margin being applied to the minimun expected strength of the bolt. The beauty of engineering is throwing out many issues as been insignificant and covered by safety margins.
The head of the bolt and the shank will not be a sharp corner and will have a radius. A hardened washer that bites that radius can be very bad practice. Bolt can fail and friction and torque values can be messed up. Spinning a washer against an alloy head would be bad. As the cap screw head landing is so small, this become quite a critical issue. Cap screws are often used on hardened steel such as die sets. With aluminum, washers are critical.
One thing that I recall from Mech Eng, is that a torqued bolt is stretched on tension and also is wound up in torsion. The net stress in the bolt is the sum of the two stresses. So one could then in theory back off the bolt a tiny bit to unwind this bolt twist. With studs it is expected that this component is smaller.
You want hardened washers to get repeatable and lower friction levels. A softer washer will not have any less elastic deformation as the modulus of elasticity is not significantly altered by hardening. Thickness will create the needed stiffness.
enough!
posted by 68.93.19...
, Tue, 5 Oct 2004 09:39:54, Tue, 5 Oct 2004 10:50:23
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