Surface finish for bolted joints

[corus]

In bolting a bracket to a rough forging it's been advised that the surface be machined to present a 'flat' surface. Most of the load is in shear and so the friction is important. In my view frictional forces are independent of the contact area and so the surface finish isn't that important as long as it doesn't cause bending in the bolts when they're torqued up. Is there a recommnded surface finish for bolted joints, and is there a cheaper method of providing a flat surface other than machining the surface?

 

[rb1957]

usually we avoid relying on friction, no?

it sounds like you want a nice smooth surface to mount the brkt, makes sense ! you might consider filling the rough surface with an expoy filler ("liquid skim"). if positioing isn't critical, maybe a soft gasket (fibrous, copper, ...).

if the brkt needs to be positioned carefully (exactly), then probably no better way than machining.

 

[Sparweb]

"flat" = "perpendicular" to the bolt hole.
Try not to assume that friction is supporting any shear load in any joint, it's too unreliable. If you need friction in the analysis of your joint, then something has been underdesigned.

The goal is simply to land the bolt head on a surface that is perpendicular to the shank, and to make the two mating parts line up their respective bolt holes in the same axis, preventing any prying or bending of the bolt when it is tightened. The simplest way to clean up the area around the hole hole is to call out a shallow spotface in the same drawing call-out as drilling the hole. The shop should do the drill and spotface operations in one set-up. As far as the mating surface of the forged part is concerned, it is considered poor practice to attach anything to a rough surface like that.

Last point: that flat surface, whether it was machined, ground, or whatever, should have been done before drilling the hole in the forging.


STF

 

[SnTMan]

corus, another consideration might be bolt load loss due to embedment. Presumably a "less rough" pair of mating surfaces gives less of it.

Regards,

Mike

 

[MintJulep]

Rough surfaces are prone to embeddment.

Embeddment results in reduction of preload.

Reduction in preload results in loss of friction force.

Loss of friction force in a joint that relies on friction force results in joint failure.

 

[rb1957]

consequences of failure ?

if it did slacken off, slightly, would it matter ? would people get killed ? would it be easy to notice and fix ?

 

[Tmoose]

for almost 2 hours a day I rely on dozens of bolted joints that rely almost entirely on friction.

 

[MikeHalloran]

In 1967, I worked in an axle factory. One of our high end car lines used a u-joint assembly with a "friction drive", e.g. four bolts, but no keys, between the u-joint and the axle flange. Apparently there had been a lot of experimentation and/or analysis on that joint, which resulted in the flange face having a _minimumm_ surface finish of 55 rms. Unfortunately, the machine tools we typically used for that sort of thing had good spindles, and could easily produce much better finishes, even with the speeds and feeds maladjusted. We developed a manufacturing "process" wherein the tool setter would install new carbide cutting inserts, then break their edges with a hammer, then run a few test parts to make sure the flange face location was correct and a profilometer said it was rough enough. I think the design has since evolved away from using friction alone to carry torque into the axle pinion. At least, I hope so.

In your case, for a static joint, you may find some suitable combination of finishes and flange stiffnesses and bolt preloads that will work for you. For a joint subject to dynamic loading, I'd want a key. But that's just me.







Mike Halloran
Pembroke Pines, FL, USA

 

[Tmoose]

Hi Mike,

i'm not saying a stout key is not sometimes a nice addition, but....

At least up until the 80s Not many passenger cars use anything but clearance fitted (despite what was published on the ARP site for a while) bolts to clamp the flywheel to the crankshaft, and cantilevered bolts/studs to hold wheels to axle flanges against torsion and sometimes even radially. For the last 3 score years a willing army of 16-24 year olds have been trying their best to violate the flywheel/crank joints' integrity and wiggle the wheels free of the axles in various creative ways. I've only known of 2 flywheels to loosen, and each time it was confirmed a torque wrench ( boo, hiss, useless, evil) was NOT used to tighten things up. Wheels sometimes loosen, but my experiences suggests those originate from installation deficiencies, not design deficiencies (except in the case of some alloy wheels that being solid lack the preload preserving belleville washer effect of stamped steel wheels, and thus require a few re-torques to compensate for embedment, etc. Still the clamping torque is what keeps things from micro-moving to death)

4 or even 8 hardened, reamed to fit dowels don't fare well when clamping friction is insufficient.

http://i292.photobucket.com/albums/mm14/turbovw408/Parts/DSCI0627.jpg

In 1979 SAE published a U-Joint and driveshaft design manual as No7 in their "Advances in Engineering" Series. A rep from Spicer U-joint division and another from Saginaw steering gear division teamed to create a short chapter on driveshaft attachment methods. The sections for each type (splines, end yokes, etc) are short and often refer to other sections in the manual. The description of flange yokes says "the yoke is held to the flange with bolts and a pilot diameter is provided to maintain concentricity. The bolt size is dictated by the clamping force necessary to withstand the forces that may result for the functional and durability requirements of a given application."

Chevy 6-s and V-8 used crank dampers located with a small key, but had to be driven onto the crank due to a cylindrical interferenc fit of about 0.0015".

http://www.crankshaftcoalition.com/wiki/images/3/3f/Sbc_damper_install_non_threaded_crank.jpg

They graduated to included a lone centralized bolt (incapable of resisting shear forces) torqued to 60 or more lb-fts thus applying 1000s of pounds of axial clamping.
Other manufacturers used a slip fit, but usually with large crank bolts tightened over 100 lb-ft.
I know of a few Volvos whose pulleys/dampers wiggled and ate the key crank, and insufficient torque was squarely behind each episode.
Early Miatas got a rep for similar loose damper failures, and each failure i read about followed a timing belt replacement (which requires removing and re-installing the damper. After a few years Mazda upgraded the crank nose. One detail was the crank bolt torque went from 85 up to 120 lb-ft. the key got longer too, but I'm confident the long key and crank would soon be slapped silly if a bolt was undertorqued.

 

[CoryPad]

VDI 2230 shows the embedment results based on surface roughness. If the bolt is stiff, the embedment results in more clamp force reduction.

Regarding joint shear force resistance, modern automobiles use friction exclusively for almost every joint including pinion flange attachments. The requires close control of geometry, friction coefficient, and clamping force, all of which are specified, produced, measured, and tested to validate the safety, durability, etc.

 

[rb1957]

"requires close control of ..." ... most of which would not be closely controled clamping a fitting onto a rough surface; IMHO.

 

[dvd]

One place to look would be "Specification for Structural Joints Using ASTM A325 or A490 Bolts," published by the RESEARCH COUNCIL ON STRUCTURAL CONNECTIONS under the auspices of AISC. This governs structural connections, but applies fairly well to mechanical connections. Section 4, Table 4.1 will provide you with a starting point to consider the type of connection.

http://www.boltcouncil.org/files/2004RCSCSpecification.pdf