How to Terminate a Tension Spring

I have to pull on a crank pin with a tension spring. The pin is a 3/8 in. shoulder bolt. As a quick experiment, I just hooked the spring over the pin, and the action worked great. Now I have to make it last for thousands of cycles. So I will put a sleeve on the pin and weld a lug to the sleeve and hook the spring through the lug. My question is, should I use a steel bushing directly on the pin, or use a larger sleeve with a bronze bushing pressed in. At least the bronze could be renewed when it wears out. Problem is, in my experience Oilites don't last long when loaded sideways like that. And I bet the spring will still wear out where it hooks through the lug. I wish there were something like a thimble for the spring to hook on so the load isn't all at one point. Seems like a weakness of tension springs with hook ends.

A loop at the end of the spring, its ID a few thousandths over the shoulder bolt OD is another possibility.

Some of the better equipment i work on very much does have thimbles in the sring ends, normally nothing more than a simple turned tube with a external grove the spring eye sits in. Bearings, use some sealed ball races or look at igus, bronzes won't last unless constantly lubed, especially in a reasonably high side loading. Oilite gets around the constant lube requirement and in my experiance will last ages in light loading environments, but at much load at all and it fails really fast! igus you at least have all the data and can plug it into there online calculators and take a pretty dang good stab at expected life spans.

If you want it to truly last google spring extension - life span, you want enough spring length so its not anything like extending all that much. The less extension as a percentage of length gains you a vast amount in life span, ergo the eye wearing out then becomes the common failure mode, hence the need for a tight fitting thimble! Its also worth adding with extension springs, you want enough stretch on em the coils are held just open, i have seen more than a few that have failed do to being allowed to fully relax and do to the natural helical motion then rub and wear there own coils.

Scag uses a spring design on their lawnmowers that may work for you. They have a lot of vibration. They don't use the coil itself to form a hook, rather they form a separate hook that has a Y-shaped end on one end and a loop on the other. The Y end is inserted into the spring coil and the coil diameter is wound smaller and smaller to wrap around the Y end. I know that probably doesn't make sense, and I really don't have a good picture of it. However,this is a picture from the parts manual that might help you. If you have a Scag dealer nearby, you might go take a look at one.

I would use a high grade plastic bushing if this is not a high speed mechanism.

Also, if you are making the spring from wire, instead of a single hook try making a coil with several turns. This is how many automotive cables were made for years. I'm sure the average heater door cable was cycled thousands of times throughout its life.

Many shoulder bolts are relatively rough on their ground diameter, if you're going to use any variant of a plain bearing I would at least polish the diameter to a higher level than as-received. I'd also use a bushing long enough to gently butt up against the crank face and under-head of the bolt (both also polished) to try to maximize dust/dirt exclusion from the journal diameter. Using a hard oversleeve for the spring to bear on is mandatory.

Especially if the direction of rotation has an "unscrewing" motion, I'd be tempted to use as low a friction ball bearing on the crank as possible, so perhaps shielded rather than sealed (I would generally prefer sealed in most cases). Even better to have a shallow "U" ground in the outer race to capture the spring loop so it can't move around.

For the pivoting end of the spring, a plain bearing is probably the best choice unless the arc of motion is great enough to allow a ball bearing (ball bearings don't like small, constant arcs). Again, do what you can to exclude dust and polish the shaft.

As mentioned, a longer spring which isn't allowed to fully relax will likely last longer, but if you have access to a variable flash speed strobe light, check the spring while operating to watch for any pulsations in the spring lenght, as harmonics could heat and damage the spring under some extreme circumstances.

Valve spring 85 rpm - YouTube (turn down or off your volume)

steve45 said:

Scag uses a spring design on their lawnmowers that may work for you...

View attachment 210113

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Yeah, we have some of these but they're too weak.
You guys are great, I like all the answers. I think I'll make an over-designed thimble out of a BB, grind 2 snap ring grooves, 2 fender washers to retain the hook.

This is a quick kluge to fix an ancient flywheel punch press that the clutch won't disengage because it won't go over center. Once the clutch opens the crankshaft is supposed to coast over TDC a little so the clutch cam completes its cycle. With so much wear, the clutch opens just before TDC and the weight of the ram prevents it from coasting over. So it hangs there with the clutch screaming. A better fix would be reset the cam on the next spline but then the flywheel would have to come off and I'm not doing that. But the spring assist works like a charm.
Naturally all the parameters are sucky -- the spring goes slack every cycle, is probably overextended, and tends to unscrew the pin. I had to do this in 2 hours with material on hand to get the machine back in production.

My favorite long-life termination for helical extension spring is a plug with external threads that match the ID of the spring. I see them on counterbalance springs on haybines. Drill and tap the plug for standard eyebolt, or make an integral eye, or whatever sort of extension you need.

My favorite story about a simple hook-end spring that failed was the centrifugal-advance flyweight springs in my first car.. Springs broke, timing went crazy, blew a spark-plug out of the head. Learned about Helicoils, 40 yr ago.

Something like the upper example work?

Don't know if it would last any better than oilite, but you could make up a bushing from cast iron and braze a ring to its side to hook the spring to. Cast iron on steel makes a pretty good bearing. You can take any nasty old casting and saw off a piece and then machine it as needed, to get the material.

metalmagpie