Saturday, Jun 02, 2012 at 22:19
Wheel studs are either Grade 8 (SAE) or Grade 10.9 (the metric equivalent of Grade 8 in SAE). Fasteners manufactured to Grade 8 or Grade 10.9 are made from low alloy medium carbon steel (commonly Nickel/Chrome/Molybdenum alloy), and they are heat treated to increase their strength and toughness.
Grade 8 and Grade 10.9 are classed as high grade fasteners, substantially stronger than regular "high tensile" Grade 5 (SAE) or Grade 8.8 (metric) - with the tensile strength of Grade 8/10.9 fasteners, up around 150,000 psi or 1034 Mpa.
When torqueing Grade 8/10.9 fasteners, the amount of torque applied is critical. The torque figures given, are usually for dry threads. If you lube the threads in any manner (anti-seize or oil or grease), then you need to reduce the torque figure by at least 10%.
A Grade 8/10.9 fastener is designed to be placed under tension that actually stretches the fastener a little, when proper torque is applied. This is known by engineers as "plastic deformation" - and the slight stretching of the fastener shank is what assists in keeping the tension on the item being retained.
If a Grade 8/10.9 bolt or stud is overtightened, this over-tightening seriously stretches the bolt/stud shank, until it can no longer apply tension under the plastic deformation principle. The shank actually becomes slightly thinner, if this happens.
You can pick this damage up by placing the jaws of a vernier caliper along the bolt /stud shank, in-line with the shank, and
check to see if you can see daylight between the tops of any of the threads, and the vernier caliper jaws.
If you find that one or more threads isn't touching the caliper jaws, then the bolt/stud has been over-tensioned, and has been seriously damaged - and should be replaced immediately.
An over-tensioned Grade 8/10.9 fastener will invariably fail when an extreme load is applied. Quite often, if one wheel stud has been over-tensioned, all the studs on that wheel have been over-tensioned.
Alll wheels and hubs should be wire-brushed on the mating areas before re-installation. Dirt, rust and
debris caught between the mating surfaces will crush when the wheel is in use, and this will release the tension on the wheel studs.
AnswerID:
487471
Follow Up By: olcoolone - Sunday, Jun 03, 2012 at 10:05
Sunday, Jun 03, 2012 at 10:05
We said but not one tensile strength is ideal for all applications depending on clamp forced, loading and shear strength plus what material it is clamping to.
As for wheel studs, if your clamping an alloy wheel the torque may only be 115 foot pound but if you clamping a steel rim the the torque may be 155 foot pounds.
As torque is determined by the bolt stretch, in an alloy wheel the bolt stretch would be less as compared to a steel rim.
So in theory if you only used alloy rims only you would be better of with a lower tensile strength bolt then what you would use on a steel rim to introduce the correct stretch and clamping force.
As for shear strength of the stud, once the rim has been secured properly; the clamping force of the rim to the hub would decrease the shear loading on the stud.
Wheel studs are a compromise and I think the mating surface of the bolt or nut has a greater part to play regarding contact surface over torque.
FollowupID:
762756
Follow Up By: Member - Chris & Debbie (QLD) - Sunday, Jun 03, 2012 at 11:42
Sunday, Jun 03, 2012 at 11:42
olcoolone, you are correct in that torque is determined by bolt stretch as this is how some studs are torqued, by measuring how much they stretch and not actual torque applied.
But I would question you reason for lower torque applied to alloy wheels. The normal reason for bolts retaining anything alloy having a lower torque is that alloy expands a lot more when hot compaired to steel, this expansion adds tension to the bolts/studs. This also explains why alloy head bolts have a lower torque than cast iron head bolts
Chris
FollowupID:
762769
Follow Up By: olcoolone - Monday, Jun 04, 2012 at 09:44
Monday, Jun 04, 2012 at 09:44
We torque wheels up in our workshop when we have to remove them and we always
check the vehicle manufactures specifications as to torque settings.
Nearly all manufactures have a different torque setting for alloy and steel rims.
Our 200 series Landcruiser alloy rims are 131Nm and the steel rims are 209Nm .... that's 50 foot pounds difference.
Most people don't think there is a difference..... alloy compressors more and has a greater chance of fracturing if placed under extreme force.
On some heavy vehicle stuff we do the the torque is calculated with degrees.
FollowupID:
762853
Follow Up By: olcoolone - Monday, Jun 04, 2012 at 09:55
Monday, Jun 04, 2012 at 09:55
On metal expansion, alloy's don't expand that much and much less then one is lead to believe under normal conditions.
As the temp increases so does the expansion.
Alloy heads on vehicle expand very little between 0 Deg. C and 130 Deg C., but go over that and they expend to a noticeable level.
The ideal temp for a combustion engine is around the 105 Deg.C mark and as an example my motor bike runs between 85 Deg. C and 107 Deg. C.
Material temp in an engine that over heats to a point of causing damage will exceed 170 Deg. C
FollowupID:
762856