Will two 1/8 thick flat washers 2-1/2" OD x 17/32" ID stacked one on top of the other and used in an assembly perform closer to one 3/16" thick washer or closer to one 1/4" thick washer in ablility to resist cupping deformation during bolt tensioning? My application is mounting a coach built body (1800#) on a truck chassis in 12 places. Fastener is a 1/2"-13 x 2-1/2" Lg fully threaded Gr 5 flange head bolt with Gr 5 flange lock nut. At each body mount location the body structure consists of double thickness of 10ga sheetmetal. The flat washers would be stacked one on top of the other with the bolt passing through the two washers and the body assembly, then through a 1/2" thick rubber pad between the body assembly and the 1/4" truck chassis upper C shape flange, a 1/4" thick rubber pad on the opposite side of the chassis upper C flange and then a 1/4" thick mild steel plate with the locknut on the bottom. The mount is very compliant, bolt is torqued to 55 lb-ft preload. Under live testing two 1/8" thick washers yielded and cupped very slightly (.020" from flat). The question is: would this result influence you to go toward the 1/4" thick single washer or toward the 3/16" thick single washer? Putting it another way, did the stacked 1/8" washers perform equal to, less than, or slightly better than a 3/16" washer will,... or more closely to how a single 1/4" thick washer will? Consider that no more testing is allowed. Should I recommend the single 3/16" or the single 1/4" thick washer to replace the two 1/8 washers?
Tek-Tips is the largest IT community on the Internet today!
Members share and learn making Tek-Tips Forums the best source of peer-reviewed technical information on the Internet!
-
Congratulations cowski on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!
Comparison of stacked washers vs single washer in assembly.
- Thread starter 19Deuce32
- Start date
Hi 19Deuce32
A sketch of your joint may help, but some further questions:-
1 why does the joint need to be torqued to 55lb/ft which is
the maximum torque for an unlubricated joint.
2 are you assembling the joint without lubrication
3 Why are you using plain washers with bolts and nuts which
have flanges already.
Normally with flanged fixings the plain washers can be dispensed with.
The site:- Bolt Science recommend not using multiple plain washers as relative movement can be transferred from nut/bearing face to washer/bearing face which can effect
friction within the joint and ultimately the final pre-load.
Have a look at this site it also contains a link to the Bolt science page I mentioned earlier.
Regards
desertfox
A sketch of your joint may help, but some further questions:-
1 why does the joint need to be torqued to 55lb/ft which is
the maximum torque for an unlubricated joint.
2 are you assembling the joint without lubrication
3 Why are you using plain washers with bolts and nuts which
have flanges already.
Normally with flanged fixings the plain washers can be dispensed with.
The site:- Bolt Science recommend not using multiple plain washers as relative movement can be transferred from nut/bearing face to washer/bearing face which can effect
friction within the joint and ultimately the final pre-load.
Have a look at this site it also contains a link to the Bolt science page I mentioned earlier.
Regards
desertfox
- Thread starter
- #6
desertfox,
I have uploaded this file to ENGINEERING.com "Body Mount.bmp" generated from AutoCad, hope it is visible.
1) Torque setting has not been altered during production of this body, the rubber pads make the assembly generally compliant. Although this is not an OEM rebound mount for traditional body on frame installations, we are trying to replicate that style of assembly using the rubber pads on either side of the frame. Space is the limiting factor. 55 lb-ft is the value for the bolt torque we thought produced the necessary compression in the rubber to get the joint to resist bounce. The rubber pads are cut from thick conveyor belt with fabric carcass and rubber both sides. The 2-1/2" OD washer under the head of the bolt is an idea to seal off a 17/32 x 1.25 Lg slot in the body structure where the bolt passes through. The slot is offset in the body mount hat shape to allow for left and right commonality in the weld assembly. The flange on the bolt head will not alone cover the slot, and water/salt will fill the cavity surrounding the bolt. We have had some instances of rust corrosion on the bolt leading to bolt fatigue failure. So there is an current production issue as well as a field retrofit issue. I would like to go to a Gr8 bolt to provide more endurance strength. That may allow us to raise the pretension on the bolt with the Gr8 but with the compliant body mount I think that the load will just add to the pretension and load the bolt further. So the 55 lb-ft torque may be used with a Gr8 if we go that way. The design of the mount in general may need to be stiffer to take full advantage of the Gr8 with a higher pretension. However, maybe even as it is now at 55 lb-ft it would be an improvement over the Gr5 bolt? Price doubles to $2 each on each Gr8 bolt vs the Gr5 at $1 each for 12 mounts total. Dacromet coating on a plain Gr8 bolt is about 65 cents/lb of bolts, on top of about $1 per bolt and might be an improvement over the Zn which does not stand up well in service. Some are advocating Zn Chromate (yellow) over Zn but I am not certain that would be much of an improvement over the current Zn. The bolt is fully threaded and currently has serrations under the head. The bolt is used in other locations on the machine so it will still be a stocked part regardless of the outcome of this issue.
2) Fasteners are Zn electro plated, no lubrication under fastener heads or on threads.
3) The idea of the large 2-1/2" OD flat washer is to "seal" off the cavity around the bolt. Up to now we have been installing the assembly without any washer under the flange head bolt, however we have experienced corrosion induced fatigue failure on some units in the north east. That is why I am interested in not having the washer cup during installation. A cupped washer would allow water to migrate easily into the slot and the cavity. I think the washer needs to be stiff and flat against the body mount hat shape. So far a single 1/8" thick washer cupped severely, so they were doubled to get a "feel" for which direction to go and as previously mentioned the two stacked 1/8 washers cupped .020". I never advocated the installation of two washers stacked, that was just to get a "feel" for which direction to go. The 3/16" and the 1/4" will be a special order large quantity purchase. So far I have made the recommendation to go with 1/4" thick and have been stopped by production who says I should have recommended a 3/16" thick washer. As of today I have not been able to laser cut some washers from 3/16" (7 Ga) or 1/4" plate to test and verify who is correct. So the crux of the situation: who has the better direction on the issue? Would a 3/16" single washer have shown to be stiffer (not cup) than the two 1/8" stacked washers, or did the 1/8" washers perform similar or even better than a 3/16" washer would have provided it would have been also tested? Meaning the 1/8" stacked washers performed almost as good as a 1/4" washer will? I thought that the binding friction between the two 1/8" washers as they slipped against each other during tensioning would make them react similar to a 1/4" washer due to the high compression in the joint. So I recommended the 1/4" washer after examination of the 1/8" stacked washers showed .020" cupping. The flange head bolt was our standard bolt and I thought that keeping it would provide additional radius of contact for the pressure distribution zone under the bolt head and then under any flat washer used. This recommendation was made with the thought also that I might need to go to a Gr8 bolt and a higher pretension, so the stiffer the washer the better was my opinion. Lots of issues in this discussion.
I have uploaded this file to ENGINEERING.com "Body Mount.bmp" generated from AutoCad, hope it is visible.
1) Torque setting has not been altered during production of this body, the rubber pads make the assembly generally compliant. Although this is not an OEM rebound mount for traditional body on frame installations, we are trying to replicate that style of assembly using the rubber pads on either side of the frame. Space is the limiting factor. 55 lb-ft is the value for the bolt torque we thought produced the necessary compression in the rubber to get the joint to resist bounce. The rubber pads are cut from thick conveyor belt with fabric carcass and rubber both sides. The 2-1/2" OD washer under the head of the bolt is an idea to seal off a 17/32 x 1.25 Lg slot in the body structure where the bolt passes through. The slot is offset in the body mount hat shape to allow for left and right commonality in the weld assembly. The flange on the bolt head will not alone cover the slot, and water/salt will fill the cavity surrounding the bolt. We have had some instances of rust corrosion on the bolt leading to bolt fatigue failure. So there is an current production issue as well as a field retrofit issue. I would like to go to a Gr8 bolt to provide more endurance strength. That may allow us to raise the pretension on the bolt with the Gr8 but with the compliant body mount I think that the load will just add to the pretension and load the bolt further. So the 55 lb-ft torque may be used with a Gr8 if we go that way. The design of the mount in general may need to be stiffer to take full advantage of the Gr8 with a higher pretension. However, maybe even as it is now at 55 lb-ft it would be an improvement over the Gr5 bolt? Price doubles to $2 each on each Gr8 bolt vs the Gr5 at $1 each for 12 mounts total. Dacromet coating on a plain Gr8 bolt is about 65 cents/lb of bolts, on top of about $1 per bolt and might be an improvement over the Zn which does not stand up well in service. Some are advocating Zn Chromate (yellow) over Zn but I am not certain that would be much of an improvement over the current Zn. The bolt is fully threaded and currently has serrations under the head. The bolt is used in other locations on the machine so it will still be a stocked part regardless of the outcome of this issue.
2) Fasteners are Zn electro plated, no lubrication under fastener heads or on threads.
3) The idea of the large 2-1/2" OD flat washer is to "seal" off the cavity around the bolt. Up to now we have been installing the assembly without any washer under the flange head bolt, however we have experienced corrosion induced fatigue failure on some units in the north east. That is why I am interested in not having the washer cup during installation. A cupped washer would allow water to migrate easily into the slot and the cavity. I think the washer needs to be stiff and flat against the body mount hat shape. So far a single 1/8" thick washer cupped severely, so they were doubled to get a "feel" for which direction to go and as previously mentioned the two stacked 1/8 washers cupped .020". I never advocated the installation of two washers stacked, that was just to get a "feel" for which direction to go. The 3/16" and the 1/4" will be a special order large quantity purchase. So far I have made the recommendation to go with 1/4" thick and have been stopped by production who says I should have recommended a 3/16" thick washer. As of today I have not been able to laser cut some washers from 3/16" (7 Ga) or 1/4" plate to test and verify who is correct. So the crux of the situation: who has the better direction on the issue? Would a 3/16" single washer have shown to be stiffer (not cup) than the two 1/8" stacked washers, or did the 1/8" washers perform similar or even better than a 3/16" washer would have provided it would have been also tested? Meaning the 1/8" stacked washers performed almost as good as a 1/4" washer will? I thought that the binding friction between the two 1/8" washers as they slipped against each other during tensioning would make them react similar to a 1/4" washer due to the high compression in the joint. So I recommended the 1/4" washer after examination of the 1/8" stacked washers showed .020" cupping. The flange head bolt was our standard bolt and I thought that keeping it would provide additional radius of contact for the pressure distribution zone under the bolt head and then under any flat washer used. This recommendation was made with the thought also that I might need to go to a Gr8 bolt and a higher pretension, so the stiffer the washer the better was my opinion. Lots of issues in this discussion.
Hi again
Thanks for the info.
I concur that a single washer is better than two, however I think your joint needs looking at from a design point of view, I am not convinced that just placing washers under the bolt head is the end of the problem.
At work now so I,ll post later.
desertfox
Thanks for the info.
I concur that a single washer is better than two, however I think your joint needs looking at from a design point of view, I am not convinced that just placing washers under the bolt head is the end of the problem.
At work now so I,ll post later.
desertfox
Hi again
You can go upto 75lbs-ft on your bolt without lubricate although that won't help the washers.
Assemblying the joints without lubricant will give a very large spread of clamping force or pre-load particularly if your only using a torque wrench. It maybe worthwhile using a better method for setting pre-load and using lubricant to reduce friction within the joint (see site I posted yesterday).
In addition by adding the washers you are altering the stiffness of the clamped components see the Roy Mech site under "Bolt Preload". Your drawing doesn't show how thick body structure or chassis components are within the bolt clamping range. I would check with your rubber pad supplier
what pre-load is recommended and also re-analyse the joint taking into account the external forces on the joint.
Regards
desertfox
You can go upto 75lbs-ft on your bolt without lubricate although that won't help the washers.
Assemblying the joints without lubricant will give a very large spread of clamping force or pre-load particularly if your only using a torque wrench. It maybe worthwhile using a better method for setting pre-load and using lubricant to reduce friction within the joint (see site I posted yesterday).
In addition by adding the washers you are altering the stiffness of the clamped components see the Roy Mech site under "Bolt Preload". Your drawing doesn't show how thick body structure or chassis components are within the bolt clamping range. I would check with your rubber pad supplier
what pre-load is recommended and also re-analyse the joint taking into account the external forces on the joint.
Regards
desertfox
- Status
Similar threads
- Locked
- Question
- Locked
- Question
- Locked
- Question
