Hi Tensile Bolts into Aluminium
Discussion
I've recently bought a brake kit from a well known manufacturer. The callipers are mounted using 12.9 grade bolts bolted into an aluminium block.
AIUI, the point of hi-tensile bolts is that you can torque them up tighter, and hence get greater clamping force. The question is, if screwing into ally, can you ever make use of this extra strength? Is the maximum torque you can apply not limited by the strength of the ally threads, which will be far lower than the strength of the bolt threads?
I guess it also depends on how many threads are involved?
Just idly wondering really, if there was any actual benefit in using 12.9s over 8.8s here, or if it's just because it looks better...
AIUI, the point of hi-tensile bolts is that you can torque them up tighter, and hence get greater clamping force. The question is, if screwing into ally, can you ever make use of this extra strength? Is the maximum torque you can apply not limited by the strength of the ally threads, which will be far lower than the strength of the bolt threads?
I guess it also depends on how many threads are involved?
Just idly wondering really, if there was any actual benefit in using 12.9s over 8.8s here, or if it's just because it looks better...
No, it simply depends on the depth of the thread, and hence the number of turns of thread that are supporting the load!
So, for a high tensile bolt, into say LM25TF alloy, you would be looking at at least 3d and prefferably 4d to ensure the bolt yields before the fastener!
ETA: the other main issue is one of thread form wear. Alluminium suffers for serious galling effects, so if the fastener is going to be installed and removed a large number of times, then usually a steel insert (helicoil etc) should be used to act as the sliding surface.
For a fastener that is fit and forget (say something into the ally block of a std engine, then steel fastener into ally is fine.
So, for a high tensile bolt, into say LM25TF alloy, you would be looking at at least 3d and prefferably 4d to ensure the bolt yields before the fastener!
ETA: the other main issue is one of thread form wear. Alluminium suffers for serious galling effects, so if the fastener is going to be installed and removed a large number of times, then usually a steel insert (helicoil etc) should be used to act as the sliding surface.
For a fastener that is fit and forget (say something into the ally block of a std engine, then steel fastener into ally is fine.
Edited by anonymous-user on Tuesday 8th November 12:58
Max_Torque said:
No, it simply depends on the depth of the thread, and hence the number of turns of thread that are supporting the load!
So, for a high tensile bolt, into say LM25TF alloy, you would be looking at at least 3d and prefferably 4d to ensure the bolt yields before the fastener!
Ah, so I got one thing right then So, for a high tensile bolt, into say LM25TF alloy, you would be looking at at least 3d and prefferably 4d to ensure the bolt yields before the fastener!
3d and 4d = ?
doogz said:
I'd imagine, although i've not really spend much time looking into it, that the worst load case for these bolts is going to be in shear.
It is considered VERY bad form to let a bolt carry shear loads directly by the material of the bolt itself! the idea is to ensure the joint has sufficient clamp load so that the shear is actually carried by the coefficient of friction between the two parts clamped by the bolt.So for a brake caliper "ear" bolted to a retaining/mounting block, the bolt torque must be sufficient to push the "ear" hard enough into the retaining block to withstand any possible shear load that could be presented to that joint. This leaves the bolt in pure tension and carrying none of the joints actual load.
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