z axis spring?

[Chip Owner]

I'm curious to know if anyone has any knowldege of tension springs.

After getting fed up propping up my Z axis when power turned off, I felt it time to look at solutions.

Limited space has ruled out an anti-backlash and I'm disinclined at this time to fit a brake, though it maybe the better option.

So the simplest solution is one or two tensions springs, I wondered if anyone else had any knowledge on springs generally or ever had to fit them?

I know the spindle etc is 10kg dead weight on the bed, according to the scales, I'm not sure if this translates to 10kg tension needed or if I just need enough to stop the drop.

Any tips greatly appreciated.

[lloydsp]

Chip,
I use an air cylinder 'balancer' on one of my two machines, because it provides constant tension on the spindle, unlike a spring, which provides little support at the top of its throw, and much at the bottom.

When I'm not using the router, I just 'park' the spindle on two 'waste blocks' to keep it up off the bed.

Lloyd

[Chip Owner]

I did breifly consider an air cylinder which are popular on some models.

Annoyingly space/design isn't available to consider this on my machine. I'm fiddling with a new dust boot that is attached on the fixed Z so that I can keep the brushes on the surface, so even less room due to bracket.

And likewise I use a length of 2x2 but again the dust boot design is getting in the way. Springs appear common on some machines, so while not brilliant should help.

[Bob La Londe]

I have three small CNC mills that came new from the factory with gas springs.  I had two of the gas springs fail under warranty and get replaced.  The third is still going strong as are the two replacements. 

The alternative in my opinion is a counter weight system, or as Lloyd suggests an air cylinder.  An air cylinder with an active regulator and a large nearby air reservoir might be ideal if a little more complex. 

Another alternative is an active off brake on the axis motor or lead screw if the motor needs no assistance under normal work loads. 

[Chip Owner]

Thanks Bob,

I did look at fitting a brake but at ths time that isn't an option. If there was a bit more structure to the fixed part of the Z axis, I may have looked at gas struts but sadly this isn't an option.

By far the simplest solution is springs and in theory widely available, minutes to fit and will work as required to stop the drop when powered down, all I need to do is find out what strength of spring would work.   I think I've found a small spring manufacturer who is open to answering questions.

[Garyhlucas]

A spring is not a good or simple option. At zero extention or compression it has zero force. Then the force varies by the spring rate lbs per inch. So there is no way to actually balance the load with the same force at all positions. A 10 kg gas spring doesn’t move until 10 kg is applied and at full travel will have about 11 kg of force. To get a similar result with an air cylinder you want a tank with 10 times the volume of the cylinder and a standard regulator and check valve that makes up leakage. It then operates just like a gas spring.

[lloydsp]

"It then operates just like a gas spring."
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Well, no, not exactly.  A 'regulated pressure' cylinder has exactly the same pressing or retraction force, no matter the 'throw'.  It'll exert (say) 10lbf at zero extension, and 10lbf at maximum extension.  (or whatever force you want, depending upon the cylinder's size and the pressure you feed it)

Lloyd