The ET Westbury "Seagull" I am building loosely represents a small river-launch engine from, say, the 1930's. Although the great majority of the original castings would have been iron, the model castings are aluminium alloy. Also the original would have been painted, but I am not going to do so - the as-cast surfaces are being hand dressed and sand blasted. On the other hand, I am going to use period-appropriate studs and nuts, rather than cap-heads bolts. The question is, for practicality and for authenticity, should I use washers under the nuts?
Washers or not?
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terrywerm
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Considering that the castings are aluminum, I believe that using washers under the nuts would be a very good idea. Others may have other opinions, but from a mechanical standpoint I would say that washers should be used, especially if you intend to run this engine.
Entropy455
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To what value are you going to torque the fasteners?
Here’s a more detailed explanation:
From an engineering design aspect, the compressive bearing stress of the aluminum under the fastener head should not exceed 150 percent of the aluminum’s yield strength. This seems counter intuitive, and I questioned it the first time I read it back in engineering school. You can in fact safely exceed 100 percent of yield (in compressive bearing force) within a mechanical joint. It isn’t until you exceed 150% of yield that the fastener head will start digging into the base material.
When I specify torque values at work, I must determine the stress on the fastener threads, the stress within the tapped-hole threads, the load-bearing stress under the fastener head (as discussed within the paragraph above), the load bearing stress within the fastener’s head/nut), the tensile stress within the fastener’s body, and lastly – the coefficient of friction of the thread lubricant used, as this can have a significant impact on the torque required to obtain a given preload.
Normally fasteners are tightened to 2/3rds of yield – that is unless 2/3rds preload will damage the joint in other ways (shearing the tapped threads, gouging into base material, or in a flanged joint - bending the flanges, etc).
I can tell you that if you are using steel fasteners within an aluminum joint, you will likely benefit from using washers. You will also benefit from inserting a heli-coil into the aluminum.
Remember that the coefficient of thermal expansion of aluminum is almost double that of steel. Thus when your engine warms up, the aluminum will expand at a faster rate than the steel fasteners. This will increase the preload on the steel fastener when things get hot. If there isn’t enough elastic region left within the fasteners, they will yield. However normally you’ll strip the threads out of the aluminum, or crush the aluminum under the fastener’s head – long before you yield the bolt/stud.
Here’s a more detailed explanation:
From an engineering design aspect, the compressive bearing stress of the aluminum under the fastener head should not exceed 150 percent of the aluminum’s yield strength. This seems counter intuitive, and I questioned it the first time I read it back in engineering school. You can in fact safely exceed 100 percent of yield (in compressive bearing force) within a mechanical joint. It isn’t until you exceed 150% of yield that the fastener head will start digging into the base material.
When I specify torque values at work, I must determine the stress on the fastener threads, the stress within the tapped-hole threads, the load-bearing stress under the fastener head (as discussed within the paragraph above), the load bearing stress within the fastener’s head/nut), the tensile stress within the fastener’s body, and lastly – the coefficient of friction of the thread lubricant used, as this can have a significant impact on the torque required to obtain a given preload.
Normally fasteners are tightened to 2/3rds of yield – that is unless 2/3rds preload will damage the joint in other ways (shearing the tapped threads, gouging into base material, or in a flanged joint - bending the flanges, etc).
I can tell you that if you are using steel fasteners within an aluminum joint, you will likely benefit from using washers. You will also benefit from inserting a heli-coil into the aluminum.
Remember that the coefficient of thermal expansion of aluminum is almost double that of steel. Thus when your engine warms up, the aluminum will expand at a faster rate than the steel fasteners. This will increase the preload on the steel fastener when things get hot. If there isn’t enough elastic region left within the fasteners, they will yield. However normally you’ll strip the threads out of the aluminum, or crush the aluminum under the fastener’s head – long before you yield the bolt/stud.
Last edited:
Thanks for the input. I have decided to use washers. The only trouble is the commercial washers I have are a bit thick at 0.025". I made a trial one today at a nominal 1/64 and it looks a lot better with the 6BA nuts.
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