Constant Force Spring failure concern

[G. Ayers (Unregistered Guest)]

I recently reinforced the Z and it got about 18 lbs. heavier, (it has 24" of travel). I couldn't find expansion springs that would work so I installed constant force springs. My concern is that they have a cycle life of 50,000 but some of my programs have that many z plunges by themselves. How do these springs fail? Do they just gradually get weaker or do they break? Any insight would be appreciated.

[kivimagi]

G.Ayers,

I'd like to piggy back on your post.

How do you tell when the springs, or coil, are no longer doing what they are intended to do?

I disconnected my z from the control box, and was able to push up the router by hand, but it took a "sizeable" amount of force. (What I mean by that, is I wasn't able to lift it with my pinky, or with one finger.. I'm really not sure what to expect though)

Thanks

[Brady Watson]

THe springs are simply for counter balance. I had 2 CF springs holding up my 5HP spindle and did a lot of 3D work for the 1st 3yrs the tool was running. The springs didn't fail, but had less pull than they did when new. They do lose spring. I replaced my CF springs with coil springs from SB.

You can get a replacement spring kit from ShopBot or retrofit kit that includes hardware, springs, spring yoke and installation instructions.

-B

[scott_smith]

My 02’ Bot had the constant force coil springs installed at Shopbot. They lasted about 3 years. They failed by breaking in half and jamming inside my Z-axis tube. I was surprised that Shopbot had gone back to the older version extension springs.

Scott

[G. Ayers (Unregistered Guest)]

I'm not sure what they are using now, I bought this machine in 1999. The springs on it were insufficiant to balance the load even before I added the extra weight. My main concern is that the springs don't bust in the middle of a long program. I do a lot of 3d work in foam. Some of the files are 250,000 lines and take over 30 hours of cutting. I run these files unattended and I would hate to arrive in the morning to a messed up part. If the CF springs last a year, that would be great.

[Brady Watson]

Don't cheese out on the springs. Get the coil spring conversion from ShopBot and be done with it. I run my SB for hours & days at a time as well while cutting and digitizing. I think I paid about $35 for the kit, but don't quote me on that.

-B

[G. Ayers (Unregistered Guest)]

I doubt shopbot can provide the correct springs. I need to counterbalance 35 lbs and have almost 30" of deflection. It is certainly worth a phone call to find out though. What is cheesy about using CF springs? Is it because they don't last? Believe me, I would rather not use them if I don't have to - a pair is over $50. Since I have so much z travel, coil springs, (even if I could find some with the right specs), would seem to do very little the higher my z is. It might be possible to link several smaller springs together to get the right specs but that still doesn't help at the higher z heigths. Am I missing something here?

[Brady Watson]

Sorry...I overlooked the fact that you have such a long Z throw. Looks like the CF springs are the way to go for now. I would look into gas springs in the future (like the ones that hold up the hatches on cars etc). They have industrial quality versions that won't smack you in the back of the head now!


-B

[G. Ayers (Unregistered Guest)]

I hadn't thought of gas springs. I wonder if they react quickly enough. I picture them as very slow moving. Great thought though. I'm going to look into it.

[mikejohn]

Anyone have any ideas for a counter balance weight, instead of springs?

............Mike

[evan]

Would you have to match the total weight of the router/spindle & Z axis to balance? And if so would the Y gantry be up to the extra weight? I guess since you can put two Z axis on a gantry it's design for the weight... The counter weight would have to be secured or riding on a rail so it wouldn't bounce around as the Bot moves around... excuse my ramblings
the idea just got me to thinking...

[gerald_d]

Could someone please explain to me what a CF spring looks like?

[stevem]

Gerald, look inside a tape measure.

[gerald_d]

Aah - that is nearly constant, but not exactly. We normally call that a clock spring.

A small diameter (low friction) gas spring with 24" of stroke will be hard to find. The bigger diameters may have too much friction at the neck seal.

You could rig a pulley and cable system so that the spring acts on the pulley - 1" of pulley travel is 2" of cable tavel.............

[pulse98]

Yes, these coiled constant force springs have a defined cycle lifespan. However, most of the time you will merely be moving the Z axis a fraction of the total spring travel. The total fatigue/strain of the spring is related to the distance that the z axis moves. So if your z axis moves up and down 50,000 time but only 1 inch per stroke, the fatigue to the spring is much less than if it moved the full axis travel 50,000 times.

I would use two or three constant force springs mounted in parallel to add up to the total goal force to counter balance your axis. One of the springs will break before the others but if designed properly, having one broken spring will not cause the head to drift downward by itself. The broken spring can then be swapped out for a good one without incident.

[Brady Watson]

ShopBot no longer uses constant force springs. Coil springs are the standard and much more reliable & resilient than the constant force springs of old...You can't fit more than 2 CF springs on a ShopBot Z axis...

-B