Is it engineering?

john robert bould · Jun 1, 2011

I was looking at the new isolastic's. The centre tube bore is one mm larger than the pin,an equiry to the supplier informed me that the "excessive" clearance allows the rear cradle to be adjusted...then tighten the bolt up.
Now being a bit thick...do i understand the rear engine gearbox cradle is held in place by the friction [Nip]the frame lugs produce on the iso adjusters? and if the grip is lost the entire back end will float about? ie the pin will slop around in the iso tube bore.... is this engineering?

rpatton · Jun 1, 2011

john robert bould said:
.... is this engineering?

We've had 'voodoo economics'. Now we have 'voodoo engineering'. If they had used a tube with 1/2" ID and it was a little tight, this same guy would probably be telling you how important it is that there is no slack in there... :wink:

rvich · Jun 1, 2011

Maybe it helps preserve the rubber not to be in contact with the vibrating part of the unit! Would make it a lot easier to get apart when it has been neglected for a long period too. And here I was busy trying to remove any corrosion from the rod.

I guess I might as well ask...is this supplier getting their parts from India?

comnoz · Jun 1, 2011

Engineering; from Royalmech engineering

Bolt loading

A bolt can be loaded in one of three ways

Tension
Shear
Combined Shear and Tension

Note: Conditions where bending loads are imposed on the bolt e.g. non-parallel bolting surfaces, should be avoided.

A bolt is primarily designed to withstand tensile loading while clamping components together. Ideally the bolt should only be loaded in tension. Any forces tending to slide the clamped components laterally should be withstood by separate means..

Holes for bolts are generally clearance holes and the best design of bolt is one with a reduced shank diameter (waisted shanks). Joints in shear depending on the bolts to withstand the shear load are not really rigid. Significant relative sideways movement must take place before the bolt shank can take any shear load (hole clearance). It is also likely that in the case of components attached by a number of bolts that one bolt would be loaded first and this bolt would have to yield before the other bolts take their share of the shear load....

Bolts taking significant tensile and shear load need to be engineered to withstand the combined stress..

In structural engineering the codes identify the use of High Strength Friction Grip Bolts (Ref BS 4604 Pts 1-2:1970). The bolts are tightened to a specified minimum shank tension so that transverse loads are transferred across the joint by friction between the plates rather than by shear across the bolt shank.

In mechanical engineering / machine engineering, items are often accurately located using dowels /locating pins. When installed these dowels /locating pins should be engineered to withstand any traverse loads. A recent innovation is to provide dowel bushings. These are used in conjunction with bolts which pass through the inside of the bushing after it has been installed. Separate holes for locating pins are eliminated. The hardened bushings absorb shear loads, isolating the bolts from these forces.

If the choice is made that bolts/screws are to take shear load the joint should be arranged that the threaded portion of the bolt/screw shank is not taking the shear.
The notes on this page relate to the mechanical engineering industry.. In the aerospace industry joints are often designed to specifically load the bolts in shear. The screws and bolts used are high specification close toleranced items and the holes are also machined to close tolerances. The bolted lap joints are generally used for critical assemblies and joints designed with bolts loaded in tension are avoided.????

grandpaul · Jun 1, 2011

In other words,

YOU DON'T WANT SLOP IN THERE!

swooshdave · Jun 1, 2011

john robert bould said:
I was looking at the new isolastic's. The centre tube bore is one mm larger than the pin,an equiry to the supplier informed me that the "excessive" clearance allows the rear cradle to be adjusted...then tighten the bolt up.
Now being a bit thick...do i understand the rear engine gearbox cradle is held in place by the friction [Nip]the frame lugs produce on the iso adjusters? and if the grip is lost the entire back end will float about? ie the pin will slop around in the iso tube bore.... is this engineering?

Who is the supplier?

comnoz · Jun 1, 2011

grandpaul said:
In other words,

YOU DON'T WANT SLOP IN THERE!

No, the bolt should not be in contact with the side of the hole anyway so slop doesn't matter. The bolt should be loaded in tension and the part should be held in place by the tension and not by the sides of the bolt. Their engineering is correct. Jim

john robert bould · Jun 1, 2011

comnoz said:
Engineering; from Royalmech engineering

Bolt loading

A bolt can be loaded in one of three ways

Tension
Shear
Combined Shear and Tension

Note: Conditions where bending loads are imposed on the bolt e.g. non-parallel bolting surfaces, should be avoided.

A bolt is primarily designed to withstand tensile loading while clamping components together. Ideally the bolt should only be loaded in tension. Any forces tending to slide the clamped components laterally should be withstood by separate means..

Holes for bolts are generally clearance holes and the best design of bolt is one with a reduced shank diameter (waisted shanks). Joints in shear depending on the bolts to withstand the shear load are not really rigid. Significant relative sideways movement must take place before the bolt shank can take any shear load (hole clearance). It is also likely that in the case of components attached by a number of bolts that one bolt would be loaded first and this bolt would have to yield before the other bolts take their share of the shear load....

Bolts taking significant tensile and shear load need to be engineered to withstand the combined stress..

In structural engineering the codes identify the use of High Strength Friction Grip Bolts (Ref BS 4604 Pts 1-2:1970). The bolts are tightened to a specified minimum shank tension so that transverse loads are transferred across the joint by friction between the plates rather than by shear across the bolt shank.

In mechanical engineering / machine engineering, items are often accurately located using dowels /locating pins. When installed these dowels /locating pins should be engineered to withstand any traverse loads. A recent innovation is to provide dowel bushings. These are used in conjunction with bolts which pass through the inside of the bushing after it has been installed. Separate holes for locating pins are eliminated. The hardened bushings absorb shear loads, isolating the bolts from these forces.

If the choice is made that bolts/screws are to take shear load the joint should be arranged that the threaded portion of the bolt/screw shank is not taking the shear.
The notes on this page relate to the mechanical engineering industry.. In the aerospace industry joints are often designed to specifically load the bolts in shear. The screws and bolts used are high specification close toleranced items and the holes are also machined to close tolerances. The bolted lap joints are generally used for critical assemblies and joints designed with bolts loaded in tension are avoided.????

I would guess the iso bolts ,housed in a rubber isolated tube on a commando being 1/2 inch are well above the sheer stress's ..So the bolts is doing what...other than preventing the engine from falling out?
It it supposed to locate the cradle unit? and if so is 1mm clearance enough to do this?
Original iso bolts where a good fit.So who is changing the goalposts on these items,,,without permission!

comnoz · Jun 1, 2011

Well neither the original or the replacement would load the bolts in shear application so it doesn't matter as far as holding the part where it belongs. The heavier wall thickness of the original part may be of some benefit in the amount of pressure it can maintain on the joint, but as you said the joint is lightly loaded so it will likely make little difference. I really doubt that the assembly positioning is that critical- I know the position of the tabs on the frame are not that accurate.

The design likely had more to do with the tubing that was handy than what would work best. Jim

Norton-Villier · Jun 1, 2011

I used a brass tube I got at the hardware store 17/32 x .014 (13 49mm x 355mm with some heat shrink that just barely slid on the brass. It worked a treat, I also put silicone lubricant on the heat shrink. The brass cam from K&S Engineering chicago IL incase your hardware store dosnt have it. I dont know the long term effects only time will tell.I got the MkIII iso from triumphtwinspares back in 2008. Just using them now.

Phil

john robert bould · Jun 1, 2011

comnoz said:
grandpaul said:

In other words,

YOU DON'T WANT SLOP IN THERE!

Click to expand...

No, the bolt should not be in contact with the side of the hole anyway so slop doesn't matter. The bolt should be loaded in tension and the part should be held in place by the tension and not by the sides of the bolt. Their engineering is correct. Jim

Ok, so why did Velocette make all there engine frame bolts a "allmost dowel" tap in fit? if side clearance did not matter ...should be a interesting answer :!:

grandpaul · Jun 1, 2011

I'm sticking with my preference, a smooth sliding fit with no slop "just because".

john robert bould · Jun 1, 2011

grandpaul said:
I'm sticking with my preference, a smooth sliding fit with no slop "just because".

Sorry my question was not aimed at you grandpaul. and your reply "smooth sliding fit" sounds good anyway.

comnoz · Jun 1, 2011

john robert bould said:
comnoz said:

grandpaul said:

In other words,

YOU DON'T WANT SLOP IN THERE!

Click to expand...

No, the bolt should not be in contact with the side of the hole anyway so slop doesn't matter. The bolt should be loaded in tension and the part should be held in place by the tension and not by the sides of the bolt. Their engineering is correct. Jim

Click to expand...

Ok, so why did Velocette make all there engine frame bolts a "allmost dowel" tap in fit? if side clearance did not matter ...should be a interesting answer :!:

No question a tight fit does position the part more accurately. That should not be how the part is kept in that position once tightened.The commonly used engineering practice is to hold a part in position with either dowel pins or shoulders so that the fastener is not used in "shear". Once the part is located and the bolt is tightened it is the clamping of the two parts that should be preventing movement. As the bolt is tightened it stretches in length and the part that stretches gets smaller in diameter. That is why the accepted way of using a bolt as a locating dowel includes a larger diameter section in the part of the bolt that is used as a dowel.. That large diameter section does not stretch so it remains a tight fit. There is that interesting enough.

Jim

rvich · Jun 1, 2011

How are they held in tension when we deliberately leave a gap at the end caps?

comnoz · Jun 1, 2011

The end caps are solid against the ends of the center tube and the center tube is what holds the tension- not the outside tube which is a part of the cradle. Jim

Is it engineering?

Similar threads