Bonded pads for bolted joint

[DerbyLoco]

Hi - I am tasked with retro fitting an access step to a cast steel railway bogie frame. Due to fatigue limitations welding is not possible so we have settled on bonding pads to the cast surface to form bolting faces which will then be drilled though and the casting tapped M12. I am looking for any experience anyone has with a suitable bonding agent. It needs to be gap filling to around 3mm with a sufficient compressive strength to maintain the preloading in the bolts without any relaxation. Once bolted up the bond will mainly see a compressive load and the climatic conditions are temperate.
We have used Loctite in the past with some success and Araldite make potentially suitable products but any other suggestions would be welcome. Railway bogies are a very harsh environment and very unforgiving of failure - I don't want these steps falling off!!

 

[Screwman1]

Here in the US we do a lot of welding on truck assemblies (cast steel) and don't have any fatigue problems with it. The only issues relate to trying to weld HT wear plate material and getting HAZ cracking. I would not ever put any type of a bonded material into a bolted joint for a safety appliance like a step- the adhesive will crack from the shock and the joint will loosen. I would go ahead and weld the bosses onto the face of the frame and then drill your holes for the fasteners.

 

[Tmoose]

If the bolts are very tight then I think I'd need many X the bolt area, and sufficient thickness of both clamped pieces to maintain any resemblance of uniform clamping pressure.

A metal filled epoxy has about the highest compressive strength.

http://www.devcon.com/UserFiles/File/MetalFilled_User_Guide.pdf

High performance full length machinery grouting uses epoxy products to "chock" the equipment to the baseplate.

http://www.chockfast.com/

 

[dhengr]

DerbyLoco:
Isn’t the primary idea of fastening safety appliances to railcar bodies and trucks, that these appliances not be fastened by welding because welding is susceptible to sudden cracking and failure without any warning. The safety appliances and steps are fairly light structural items and highly susceptible to vibration and thus fatigue at their connections. Rather, these appliances should be fastened by means less susceptible to sudden failure when used or loaded as intended. Lock nuts and bolts, huck bolt type fasteners, rivets, etc. which might even loosen a bit, over time, and give forewarning of need for replacement, but still not fail when loaded. I’m not sure that your bolt and tapped hole in the steel truck casting meets this locking intent. I would be inclined to do some light grinding, of any high spots, on the steel casting to start to get good bearing points/surfaces for your sill step, and then bolt it directly to the truck frame. A question I would have about any fillers or bonding agents, is their long term compressive strength and potential for creep under load. Also, their deterioration under normal environmental conditions. If I were to use any filler in these joints, I would want any which was not just a low spot filler to squeeze out during the bolt-up, so that the majority of the contact was steel on steel.

 

[tbuelna]

I do not know of any adhesives that will not creep over time when subjected to sustained loading.

There is also the obvious question of how you will ensure a compressive preload in the adhesive bond joint without any other mechanical attachment. The only benefit to applying adhesive to the clamped interface between the pad and casting would be to increase its shear capability by creating a much higher coefficient of friction with the adhesive.

 

[DerbyLoco]

Thanks for the very helpful replies.
Screwman1 - It is a welded frame but we have dismissed welding the step on as it is a retro fit and we are concerned about weld quality being inadequate and also about sub surface defects being revealed.
Tmoose - Hopefully the Stress Analyst has got the bolt calculation right! Both the products you suggest look very promising, especially the Chockfast.
Dhengr - All your points are completely valid. I am looking for most of the load path to be through metal to metal contact with the filler taking up the gaps although I fear we won’t get a flat enough surface every time. Given careful fitting we might be OK but I am not sure that can be relied so I think there will be some bond compressive loading. I cannot get at the back of the joints hence the screws which will be high strength Loctited, (maybe wired as well to stop them falling out if they get loose). You are spot on about light structure on bogies - we have proposed the first installation is instrumented to check for unexpected resonances which have given serious problems in the past.
Tbuelna - I am afraid you may be right about creep but we have used bonded pads before on new builds under spring seats with success and they are compressively loaded although they don’t see much shear.
In conclusion it’s not a very satisfactory solution but I hope it can be made to work as there has to be a step and the existing body mounted version falls foul of health and safety rules, (not UK ones for a change!).

 

[kingnero]

Cannot you mill the surface? I suppose that isn't feasible?

Contact a local welding engineer, it might not be such a bad idea to weld after all. If done correctly, the fatigue issues for a fairly light-weight structure shouldn't be insurmountable.

I would also be very leary about using a filler inside the bolted connections. Either way, I'd prescribe regular inspections...

 

[Tmoose]

"unexpected resonances which have given serious problems in the past."

My impression of this installation is you are adding human sized steps to something like this -

http://webzoom.freewebs.com/tuscanriverworks/textures/bogie2.jpg

Creating/invoking a local resonance with a relatively small device bolted on to a load bearing frame not connected to, or in the vicinity of a high speed turbine or gearbox would not be something I'd have expected.

On the other hand If the contact surfaces were uneven or worse all in a straight line to allow pivoting and bolt bending and the bolt clamping was inadequate (either by design or as a result of installation) especially if spring type lock washers were involved then I would not be surprised if over time things loosened up.
"The strength of most highly loaded bolts and studs is determined by the man with the wrench and not by the designer, the metallurgist, or by the manufacturing processes. " J.O. Almen ~ 1944

http://www.shotpeener.com/library/pdf/1944000.pdf

("long" bolts as always can provide protection from preload loss by their stretched installation)

A wide spread 4 bolt pattern and a backing plate intentionally thin enough to be flexed by the bolt clamping to conform to the casting could be pretty secure.
Or add a 25 mm dia spot face the casting at each of the widespread tapped holes and install the semi-stiff backed step on short stand-off spacers.

 

[DerbyLoco]

Hi Tmoose - That’s a big bogie. Ours is under a DMU running a 16t axle load at 160km/h with rubber suspension and you can pick up a lot of resonances. Things have fallen off similar bogies before and needed a lot of design input to solve.
I am having a re-think about getting the pads more level but I am stuffed for doing any on frame machining - it’s just not practical and welding is definitely not a possibility either.
I really appreciate the help given so far -thanks everyone.

 

[Tmoose]

Hi DerbyLoco

How are you creating the M12 tapped holes?

A pretty big spot face tool can be done by hand with relatively little drama if it has multiple cutting flutes.

http://portermax.com/index.php?main_page=product_info&products_id=66

Creating 3 or more spot faces in ~ the same plane would require a little finesse, but could permit using equal height stand-off spacers.

A mag base drill is less scary, (I assume the bogey is ferrous, and magnetic) and probably better for the tapped holes anyhow.