Date: Tue, 10 Dec 1996 17:33:15 -0800 (PST)
From: Mike West <mwest@cdsnet.net>
Subject: Head Studs and numbers


 You're about to enter the fascinating and exciting world of screw-
threads.

 As I recall they were invented by Leonardo da Vinci.

I was just a boy at the time. A beardless youth.

He also invented the Helicopter and the Machine gun.
 All the important stuff.

 He was way too cool to be from the era he came from.
I figure he was either an alien from another planet or
from a future where time travel is a fact.
 The last is less likely since I expect we have no future.

 Screws  are fascinating because of all their uses.
You can add, subtract, multiply and divide.

You can measure stuff. You can push or pull.
 Clamp stuff together or drive them apart.

You can pump water or anything else that will lay there.

Drive the mightiest ship thru the ocean or the largest 
airplane thru the sky.

Probably stuff I never even heard of. 

 It's uses are only limited by our imaginations.

 They talk of "better living thru modern chemistry".
Try living without the noble screw!

Last but not least you can take two totally unrelated things
and screw them together and make one big thing!

And if we want to we can reverse the process and make it back
into two parts!

 Absolutely mind boggling! The Engineers dream come true!

 This type screw is called a "Fastener". The name has about as 
much class as "wrench" or "horsepower". Or "fart".

 This was brought about because we had to sell them to the farmers
and sundry other earthy types.

 You start talking about "tranverse compound rotary motilators" and
you lost them.

 What they do is fasten stuff - fast, and hold it "fast".

 So that's what we're going to use them for.

 We need a device that will cunningly fasten two parts together and
simultaneously hold a third part between them with no physical
means of support.
 
 We're going to screw our heads onto the cylinders by way of the 
engine case and effectively we will have one strange shaped part
made of three parts, that vibration and pounding and shaking nor wind
nor sleet or snow nor dark of night will make it give in.

 The Loyal and Noble Screw.  'Nardo, you devil you!

 Definition: a screw with no head is a "threaded Stud".

 Unless it's a propeller or some other marvel form of screw,
but we're talking "studs" here.

 In our "small wonder" vehicles there are 8 of these to a head.

 They are 8 mm on some and 10 mm on later years. When this change was 
made I don't know. In 1973 the factory also installed steel inserts in
the case for additional strength. I'm not the historian.

 Looking at the 8mm (5/16"), the manuals say "torque" to 18 ft-lb.

 Ok that's what we put on the wrench but how much "clamping" force
is that? 
 
 According to some numbers I heard there's 3000 lb trying to get out 
of that head on my stock buggy!

 There's a "not so nice" formula for that.
It has all these greek letters and symbols and looks quite 
impressive and also quite intimidating.

 It includes things like the lever arm and the number of
threads and the angle of same minus the coefficient of friction 
and the etc. etc.

 We have long ago found a way around this nasty formula.
  It's called "tables".

 Looking at the table for threaded fasteners, we find, there it
is, 5/16 should be torqued to 18 ft-lb.

 The text with the table says it is based on 60,000 psi and 80% 
of the yield strength of the material.

 There is a reason for the notation.

 In addition to it's duties as a "fastener" the stud also 
functions as a "Spring"!  

 It just never ends, does it?

 Alright, if we had a condition where it was just going to lay 
there, you could just tighten it down with your fingers and it
would stay there.

 We have a condition far from that.
Vibration, temperature changes, pounding etc. all add up to bad
news.

 Do you know what the most elastic material on earth is? Steel!

So we run that fastener up into it's elastic range, between 60-
80% and it will stretch and shrink right along with the parts.

 Is this amazing stuff or what?  :-)

 Now we get to look at the force being applied to the head by
the stud.

 The 60,000 psi is "per square inch".

So all we have to do now is figure out the square inches of the
fastener cross-section and multiply that times the 60k and we have our number. 

So, 8mm is .32" (it's the colonial thing again), the formula
for area is "diameter squared times pi over 4".

 Plug that in and you get area equal 0.08 sq.in.

 .08 times 60,000 is 4,825 lb clamping force.

 Fooled you on that one didn't we?  :-)

 Each one of the four fasteners around that piston chamber is
quite capable of holding the head down and we have 4 per cylinder
or 8 per head.

 All from your effort of 18 ft-lb of torque.

 Now you want to know why so much. Or "he's full of it".

 There is another factor and that is "crush"
of the sealing surface of the head to the cylinder.

 I have no data on the head alloy, remember?

 Then there's Cyclic loading. We are deep into the realm of
 "cyclic loading".

 Another factor is "Lame mechanic".

 The best way to describe cyclic loading is with an elavator.

 "Elevator ?"!!

 The law requires that the cable on an elevator have a "safety
factor" of 11 or more. Which means the cable must be 11 times 
as strong as it's maximum load.

 Because they break if you don't. Cyclic loading.

 Just people getting on and riding to the next floor and back 
down.

 The engine is pounding on the head a couple thousand times a 
minute.

 Discounting "Crush" and "Lame", we have a safety factor of
 6.4.   That's the stock 1600 with 7:1 compression.

 The weak point of the stud setup is not the stud but the case
itself.

 You recall(of course you don't) the case has a yield strength
of only 20,000 psi.

 So VW went to 10mm and inserts for a larger diameter.

 That about covers it. 

 I hope some of you got this far!   :-)

 west