Need an engineer's help

[Ronald W.]

What is the difference between a 50 pound brake shoe return spring and a 40 pound spring ? Is it the force required to stretch the spring to its working length ?

[Mark F.]

Re: Need an engineer's help

Ron, Your probably new to this site based on that question. Point being you will get a bunch of wise guys responding with the answer "10"....daaa.

Good luck with you answer.

Mark,

For what its worth, I think the answer is that the spring is capable of producing that much force installed.

[Duke W.]

Re: Need an engineer's help

What is the difference between a 50 pound brake shoe return spring and a 40 pound spring ? Is it the force required to stretch the spring to its working length ?

Yeah, ten pounds! Who needs an engineer?

Your analysis is probably correct. Return springs work over a very narrow range of displacement, so the return force they apply is essentially constant whether the shoe is retraced or pushed out against the drum by the wheel cylinder, and I doubt the extra ten pounds would be very noticeable at the pedal even with manual brakes.

Duke

[Joe C.]

Re: Need an engineer's help

What is the difference between a 50 pound brake shoe return spring and a 40 pound spring ? Is it the force required to stretch the spring to its working length ?

Springs are rated by the amount of force necessary in order to effect a change in length. In this case, pounds-force/inch: the force needed to elongate the spring one inch from its free length.

[Douglas C.]

Re: Need an engineer's help

What you're referring to is known in engineering terms as the spring constant.

F = K x d,

where K is the spring constant and d is the distance the spring is displaced when the the Force, F, is applied.

If it ain't math, it ain't engineering.

--Doug (Ph.D., Aeronautical Engineering)

[Bill M.]

Re: Need an engineer's help

Joe and Doug:

50 lb/in? I remember them as being much stiffer than that!

Bill

[Chuck S.]

Re: Need an engineer's help

...If it ain't math, it ain't engineering...

Aww...peshaw, Doc, engineering is about a lot more than math! Remember that the origin of "engineering" is from that little old guy in greasy overalls and flop hat that drove nineteenth century steam engines.

[Duke W.]

Re: Need an engineer's help

What is the difference between a 50 pound brake shoe return spring and a 40 pound spring ? Is it the force required to stretch the spring to its working length ?

I've never seen any rate specifications for brake return springs, so I'm interested in more details. Who's the manufacturer of the springs and where did you get the specs. Are these springs for Corvette drum brakes?

It's been awhile since I removed or installed a drum brake return spring, but it seems to me that the rate is a lot higher than 40-50 lbs/in. Many non-engines say a spring is "X-pounds", when they really mean pounds/in, but in this case maybe the rating is actually the working force when the spring is installed.

So give us the whole story.

Duke

[Ronald W.]

Re: Need an engineer's help

I am trying to distinguish the difference between the gray 40 lb primary brake shoe return spring and the black 50 lb secondary return spring. Unfortunately the sets are no longer available from GM.Iv'e weight tested many sets of black and gray repro springs and found the individual springs vary as much as the sets. The difference between a 40+50 pound spring is only 3/16 of an inch. I need to know how an engineer would identify the correct spring so I can select individual springs that would meet the original specs.

[Duke W.]

Re: Need an engineer's help

Actual rates on these types of springs can vary widely from nominal, so a 40 lb/in might be within tolerance (or barely out) of the 50 lb/in and vice versa.

Where did you find these "specs"? Are they lb/in rate or pounds force at some specified length?

Duke

[Bill M.]

Re: Need an engineer's help

Ron:

Both springs are about 0.5 in diameter
Both springs have about 20 turns
The gray are 0.100 wire diameter
The black are 0.106 wire diameter

Bill

[Ronald W.]

Re: Need an engineer's help

Thank you engineers. Problem solved. Brake service procedures from Corvette service guide st-12 are directed to 1961 Chevrolet shop manual. Page 6-8 Note; Be certain that the primary and secondary springs are kept separate as they have unequal tension. The secondary spring which is colored black has a spring rate of 50 lbs. The primary spring which is colored grey, has a spring rate of 40 lbs.

[Jack H.]

Re: Need an engineer's help

Thank you engineers. Problem solved. Brake service procedures from Corvette service guide st-12 are directed to 1961 Chevrolet shop manual. Page 6-8 Note; Be certain that the primary and secondary springs are kept separate as they have unequal tension. The secondary spring which is colored black has a spring rate of 50 lbs. The primary spring which is colored grey, has a spring rate of 40 lbs.

My previous engineering instructors would give this a grade of ZERO for improper units. Has been one of my pet-peeves ever since. A spring rate CANNOT have units of lbs. As Doug has mentioned, spring rate has units of force (e.g. pounds) divided by displacement (e.g. inches). Maybe as someone mentioned, this is the installed FORCE based on the displacement in installed configuration?

Another good example of improper units that kills me is when someone says they run "35 pounds" in their tires...no, you run 35 psi (pounds per square inch).

Mr. Kuhlman would be proud of me right now...I'm sure the rest of you think I'm a geeky nut.

[Bill M.]

Re: Need an engineer's help

Thank you engineers. Problem solved. Brake service procedures from Corvette service guide st-12 are directed to 1961 Chevrolet shop manual. Page 6-8 Note; Be certain that the primary and secondary springs are kept separate as they have unequal tension. The secondary spring which is colored black has a spring rate of 50 lbs. The primary spring which is colored grey, has a spring rate of 40 lbs.

I remember that from the shop manual from decades ago at which time I thought: no way are those springs that soft. I don't know what 50 and 40 are, but they're not spring rates.

The gray spring calculates at 112 lb/in
The black spring calculates at 142 lb/in

I thought that was too soft, so I measured a black at 166 lb/in. Close enough.

Now I have time to play with this stuff...gotta love retirement!