Lotus Elan

Donut bolts

PostPost by: chrishewett » Sun Aug 07, 2005 8:25 pm

I have just had another failure of a donut bolt. I assume that it broke and fell out as it had a new nylock nut on it. I bought these bolts from an agricultural engineer and was assured that they were high tensile. Are there different spec on high tensile bolts? If so what should they be.
I recently changed the driveshafts to a system with one donut inboard and a uj outboard. It was one of the bolts connecting the donut to the diff output that broke.
Any help would be appreciated .
Chris
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PostPost by: cliveyboy » Mon Aug 08, 2005 7:51 am

Chris
Where do I start on the subject of bolts.
Yes there are many different grades of bolts, the trouble with specifying high tensile is that you also need to state how "High Tensile" you want the bolt to be. The most common "high tensile" bolts are usually grade "8.8" The first 8 means that they will take 800N/mm2 and the second 8 means they yeild (stretch) at 80 percent this is when the clamping force will reduce (bolt or nut comes loose)the bolt has not broken only stretched. The other common grade is 12.9 which is 1200N/mm2. These grades are used on metric and newer type bolts, how ever you also get lower grades which are also classified as high tensile some are weaker than certain stainless steel bolts. Never use stainless steel on stressed components i.e Rotoflex bolts,engine mounts, suspension etc
For older type bolts they are marked with a letter usually "S" or "T"
This denotes the strength of steel they are made from "T" is similar to 8.8
(When I machine engine studs for my mates race cars I use "T" grade steel).
How ever the strength of the bolt is not the only crucial factor.
Bolts and Screws:- There is a difference a bolt has a plain unthreaded part to it whilst a screw (proper name set screw) has the thread all the way along its length. Bolts are designed to take all the load across its plain unthreaded part such as in the rotoflexes. The plain shaft must go all the way through the component otherwise you start to load on the thread which has a smaller cross sectional area. To give you an example if you use a 12mm screw instead of a bolt on a drive shaft the smallest cross sectional area across the threads is approximately 10mm diameter. this would only gives you the same strength as using a 10mm bolt therefore by just using the wrong fastener you have seriously weakened the joint.
Set screws are normally used for stopping things pulling out such as anchoring something to the floor where the load is traveling the length of the screw as opposed to bolts in a drive shaft which are trying to shear across the diameter.
The bolts for the rotoflexs are special (NAS bolts) as they have specific plain lengths on the shaft to go all the way through the rotoflexs and are also a much higher grade/strength.
Hope this helps and I have not gone on too much.
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PostPost by: elansprint » Mon Aug 08, 2005 5:16 pm

The donut bolts are a special bolt using an ordinary 8.8 ht bolt is no good the donut will fail

regards

Ian
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PostPost by: nebogipfel » Mon Aug 08, 2005 7:21 pm

Chris,

Susan Miller has a good stock of the correct bolts
John

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PostPost by: rgh0 » Tue Aug 09, 2005 12:13 pm

Bolts and bolted joint assembly are a complex topic and if you want a good practical guide for automotive use I suggest you get and read the 223 pages of Carroll Smiths "Nuts, Bolts Fasteners and Plumbing Hnadbook". True understanding of bolted joints under cyclic loading requires an engineering degree and a lot of materials science and mathematical analysis.

The orginal Lotus bolts in the donuts were SAE grade 5 equivalent in terms of material strength ( UTS of 120,000 psi). The 8.8 designation is a metric designation not a SAE / UNF one but is similar in terms of material strength. Thread lengths versus plain shank length and bearing surface tolerances were more special (i.e to NAS or AN specification or equivalent ) rather than the SAE standard to keep the threaded section outside the shear points on the assembly and the bolt not under bending when tightened. While this is good design practice and obviously required by Lotus in their orginal standard design and bolt supply and also recommended by Carrol Smith in his book, in actual use over many miles of donut use I found this non critical and standard SAE grade 5 bolts of the right overall length worked OK for me.

What is critical in my experience is to use at least a real SAE grade 5 spec bolt or better ( 3 radial lines on the head), rather than fake and there are lots of fakes around. You then need to torque it correctly to ensure the bolt does not see cyclic loads, the best bolt will still fail if not torqued correctly due to fatigue.

If you can source bolts to the original specification easily from the Lotus suppliers and you are confident they do match the original quality then this is obviously the best and easiest solution rather than trying to spec them yourself if you are not familiar with all the issues around specifying a bolt correctly.

regards
Rohan
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PostPost by: chrishewett » Tue Aug 09, 2005 7:32 pm

Thanks for all the replys.
Obviously this is a more complicated subject than I had thought. My problem is more complicated because I have fitted a non standard drive shaft. I bought these from ebay and have no idea who produced them so I can't ask the makers. The bolts I bought were from different suppliers. On the outboard end I used bolts obtained from Chrisneils. They are shiney and have R S B and S on the head ( S grade?). I think that these would be ok but as I have no donut on that end the unthreaded part of the bolt is to long . On the inboard end I have a donut and also an extra piece which holds the shaft if the donut fails so they need to be longer than the originals. The bolts I used there are from an agricultural supplier and I dont think they are up to the job. I will now get underneath and try to accurately measure the dimensions I need and take it from there.
Chris
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PostPost by: nebogipfel » Tue Aug 09, 2005 9:42 pm

Chris, Your shafts sound like the Spyder design.

Go here for a pic'

http://www.spydercars.co.uk/driveshaft_kits.htm


Perhaps they could help with replacement bolts?
John

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PostPost by: Hamish Coutts » Tue Aug 09, 2005 9:55 pm

Chris,

I would say that the bolts ARE the spyder design. In my dealings with them I would have thought it well worth a phone call to them to get the correct bolts sorted out. The only other option I can think of is TTR.

Best of luck,

Hamish.
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PostPost by: Hamish Coutts » Tue Aug 09, 2005 9:57 pm

Sorry Chris,

I meant to say that the DRIVESHAFTS are the Spyder design. (must be too late at night)

Regards,

Hamish. :lol:
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PostPost by: poiuyt » Wed Aug 10, 2005 1:35 am

I got mine at Dave Bean.

Steve B
Steve B.<br>1969 Elan S4
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PostPost by: Rob_LaMoreaux » Sat Aug 13, 2005 12:59 pm

Use Aircraft grade bolts.

These are what Lotus used on the "special" bolts since they have a longer shank so that the shear load is on the shank not the threads. The proper bolts for the rubber donuts are:

Driveshaft to donut AN7-31A
Halfshaft to donut AN7-25A

Rob
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