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Gerald D Fri 14 September 2007 09:08

Weight balancing springs - Gaslifts and coiled wire
4 Attachment(s)
I deleted the original version of this thread by mistake - here is a re-creation:

When the stepper motor is switched off (the Gecko's power is switched off) it has some resistance or friction that we call the "detent" torque. This detent torque is not enough to hold the mass/weight of the router/spindle and it needs some help. The spring gives the rough help. The spring is not an exact balance of the mass - it is only an assistance to the detent torque.

A motor with gearbox or belt reduction has a high detent torque and then the "accuracy" of the spring is even less important.

When the motors are switched on (power to the Gecko's) then the motor locks into a firm position and will hold the z-slide up, even if there are no springs. The funny thing about this is that you only need the springs when you don't need the MechMate! (Or, if the power trips, or you hit an e-stop:eek:)

All the above is to explain that this spring issue is not an exact science. If your spring is too strong, the z-slide will ride up when you switch everything off. That is better than having a too weak spring where the cutter drops into the table.

A system with a wire coil spring should hang about in the middle of the z-travel when the motor is pulled away from the rack.

For "Gaslifts" in USA you can look at part 9416K18 from, but you have to specify a force. The gauge plate weighs 2.4kg, the 50x25x3 tube weighs 1.5kg, the bit of rack weighs 0.4kg...that is about 4.5kg [10 lbs] total. My spindle weighs 5kg [11 lbs) that is 9.5kg [21 lbs] altogether so I would have to specify a 20lb gas-lift from McMaster. You are going to use a router, so you have to go for the 15lb option - there is no other choice.

The best spring that I can select from McMaster, for balancing a 6 lb router with a 10 lb slide, is part no. 9664K27. That is actually a parcel of 5 springs each 36" long! You need to cut a single spring only 7.3" long from all of that. A single pre-cut spring is not much cheaper, and then you can't play with the final balance - get the long stuff and then you can experiment.

Art Ransom asked: My Z slide assembyy weighs 31.5 lbs and has 8" of traved. What size if gas slide should I use?
30 lbs with 10" of travel?

I said: That sounds about right

Harold Weber asked:
Gerald, last November you said you were planning to replace your gas spring with a coil spring. What is the reason for this? I looked up this thread with the thought that I'd like to try the gas spring, now I see you are changing???

I replied:
Harold, I think you misunderstood what I said at that time. Without looking for the exact post, I believed I tried to say that I might need a coil spring in the future as a backup, in case the gas spring didn't work out. I had been nervous about the gas spring from the start because it is not recommended for oscillating movement. (Gas springs always seem to be used in fairly "static" applications, never dynamic). However, the results so far have been very good, and if this one loses its gas now, it will be replaced with another gas spring.

I still owe everyone a design based on the coil spring. The bottom of the coil spring will attach to the bottom of the z-slide - that's very easy. Then there is a little piece of the spider plate to cut off - the anchor point of the gas spring. But the problem lies in attaching the top of the coil spring to a sky hook (or drip stand) above the Y-car....

Rigging up a hook is easy - making it look pretty is another thing. And then it should also be used for holding a future dust hose.....and foot...which must be height adjustable.... There are still a lot of small details to be drawn onto paper.

Harold Weber asked:
Now then, is there any way to find a post that I made on your old Forum? I made only a few posts there, and in one of them I think cited a gas spring source that claimed they had optional special rod seals available for dusty environments (or something like those words). I cannot recall the name of the manufacturer but I'm pretty sure I put it in the posting. If you could get me the name of that company, I'll be sure and buy you a beer the next time I see you:D

I replied:
One beer on ice please: link :D:

"The gas spring sounds like a good idea. I found this web site that discusses how gas springs work. They mention a "scraper" option which can be used to remove sawdust. They also mention service life of 50000 to 70000 cycles. Not sure how that would apply; I'm thinking that I could get a couple of thousand 10 mm movements up and down while cutting some 3D parts. Would not be using the full stroke very often though..."

The scraper seal mentioned there is typical for pneumatic & hydraulic cylinders. We know them as "rod wipers" or "rod scrapers", available from all seal producers. But I've never looked for them that small. Should be easy to fit an after-market seal to any old gas-spring, if available.

Our gas spring has done quite a bit of 3D texturing, but the step-overs are mostly about 20mm plus.

Hugo Caradini asked:

Hello Gerald and friends.
I am almost done. I was stop waiting for my cable tracks and my cables but they came in today. My mechanics is ready except for the spring :confused:that substitutes the gas lift in the Z axis and I guess you will post something about it. My electronics are ready to. I Just have to make the cable installation and a case for my monitor and computer. Gerald this machine looks great, thanks for your patience. I will be posting soon after I get back my camera that went to a trip to El Salto Angel and Canaima:)

I replied:

Some ideas for Hugo:
Attachment 338
Attachment 339
Attachment 340

That was easy to attach the bottom point of the spring.

But the top point needs to be something like the red part shown below:

Attachment 341

Sorry that I cannot think of nice details for it.

Art Mon 08 October 2007 10:38

Z axis motor help
I have been using a 640 oz motor with a 30 lb gas shock on the Z and it works fine. I want to extend the travel and the simplest way to solve the problem is to elimate the gas shock but that means that the motor will have to be able to hold the weight of the 30 lb Z axis with power off. A spring can replace the gas shock to assist. Will the PK296A2SG10 ( 10:1 ) provide enough holding power? Any suggestions? Orential has a stepper with electric brake but the $1000+ price range kills that.

Richards Mon 08 October 2007 11:18

Art, the rotor inertia of that motor is 7.7 oz*in. So, even with a 10:1 gearbox, the motor would 'hold' less than five pounds. Have you thought about using a counter-weight of some sort?

Art Mon 08 October 2007 14:17

Counter weigh
Have to work on that if the idea of an electrinic brake falls through. There is an electronic brake that is designed for stepper motors and I am trying to find out more details.

normand blais Tue 09 October 2007 07:22

Hi I was also thinking counterweight and if a hydraulic line like brake line connected to z and the weight on the outside

Gerald D Tue 09 October 2007 09:17

Art, if the gas shock worked fine for the short stroke, why don't you get a longer gas shock?

Hi Normand!

Art Tue 09 October 2007 11:15

I coundn't see the forest for the trees
When my machinest made the Z plate he made it 24". What I had in mind was that I didn't want to move the A axis up and down to be able to do different diameter pieces. This reqired a stroke of 12". The Z plate I have will allow a 12" stroke but the gas shock only has a 7" stroke. A gas shock with a 12" stroke would require a lot of rework to mount it. I finally realized that 90% of the time I will be doing turnings less than 12" and flat routing. This is using the Z in the bottom of it's working length and putting the gas shock mounting stud at the top of the Z plate. When I need to do anything larger than 12" I just move the mounting stud down and this moves the down limit higher from the center line of the A axis. Problem solved by drilling and tapping another hole for the gas shock.

Gerald D Tue 09 October 2007 11:24

It is amazing how often a technical problem can be solved by drilling a hole!

smreish Tue 09 October 2007 11:31

...Now that is a quick and easy solution. Just requires the operator to make sure the set up is correct! Love that solution.
I was actually going to suggest making a trunion spacer plate (like you use when you lift the suspension on a truck) to move the trunion connection lower.

Drill and Tap MUCH easier.


Gerald D Tue 09 October 2007 11:50

1 Attachment(s)
The MechMate z-slide can accommodate all the following McMaster-Carr struts in terms of outer diameter:

Attachment 455

The upper mounting, to the rect. tube of the slide, is the thing to be worked out . . . . . .

Gerald D Wed 10 October 2007 00:47

1 Attachment(s)
The new drawings are based on the 9416K2 from McMaster:

Gerald D Wed 10 October 2007 00:56

If I havn't said so before . . . .

Do not tighten the nuts at the ends of the strut after installation. The strut's own force keeps it in place, the nuts are actually not needed at all. If you don't fit nuts, somebody is going to come along and say, "Hey, the nut has come loose and fallen off - let's fit a new nut and tighten it hard this time!".

When you tighten the nuts, it could put a side force on the rod seal, and that is the last thing you want. So I suggest either:
- nylock nuts just to cover the threads, or
- grind the threads off.

IN-WondeR Sun 01 June 2008 02:44

How do you calculate the force that the gas spring for the Z-slide need to be.??
I have weighed the parts that is part of the moving z-slide and they weigh in at around 8.5 kg's

Would this mean That I would need a Gas Spring that has a pulling power of 8.5kg's.???

Gerald D Sun 01 June 2008 05:10

Yes, that is nearly correct. A gas spring pushes with a nearly constant force (quite a lot of 'stiction'/friction)

J.R. Hatcher Sun 01 June 2008 06:36

Kim I thought it was rocket science :rolleyes:but real soon realized it is not. The Z stepper motor like the others is capable of moving a relative amount of weight. Lets say you were able to adjust the weight of the Z from 30 lbs to 10 lbs with a gas spring, this would be acceptable. Even if you were over like you used a 40 lb gas spring with a 30 lb Z it's just the same weight only in the opposite direction, this too would be OK. The thing that confused me was gas springs do not actually read what they are rated for. In other words a 20 lb gas spring will read 30 lbs on the scales???:confused:
We need other opinions here.:cool:

IN-WondeR Sun 01 June 2008 06:44

So if I use a Gas Pring that is rated at 100Nm and I have a Z-Slide weighing in at 8.5Kg = 85Nm the It would work fine, and I could even add some xtra weight to it if needed.. I have not added the weight of a dust collection unit so Perhaps, I would even need something more than 100Nm, I might as well Ad 20 more while I'm at it...

IN-WondeR Thu 12 June 2008 05:50

I have just found these on a british site...

They can be adjusted to fit between 10kg's and 65 kg's...

And at a fair price... Do you guys think these would be good to use.???

Gerald D Thu 12 June 2008 06:42

In principle they look okay. The function should be as good as the non-adjustable type.

Gerald D Thu 12 June 2008 06:43

I am getting slightly surprised that our first gas spring is still working fine after 2 years - was originally thinking that I would be lucky for a year at a time.

IN-WondeR Thu 12 June 2008 06:57

It's not easy finding something here in Denmark. And mcmaster will not ship outside the US to new costumers, so there I can't order one from...

Kobus_Joubert Thu 12 June 2008 22:41

I bought mine from a motorcar spares centre.

liaoh75 Mon 30 June 2008 17:09

After reading through all of the Z-slide gas spring posts, I didn't see any distinction between the correct gas spring for the 350mm (14") slide and the 250mm version. I would like to build with the 350mm (14") z-slide option for future flexibility. Would the recommended 9416K2 from McMaster work or should I select one that has a longer stroke? Can someone give me advice me as to the correct McMaster part number I should be using or let me know what stroke length I need?

Gerald D Mon 30 June 2008 20:51

The stroke of the rack and the slide plate is 350mm [14"]. You need a spring that matches this.

9416K54 looks like the nearest McMaster spring. I don't know if anyone has tried it yet.

Gerald D Tue 01 July 2008 00:01

Have checked some notes and found that I did check the design against the McMaster 9416K54.

liaoh75 Tue 01 July 2008 00:15

Gerald, thank you for the reply. I looked at the McMaster 9416K54 gas spring, but noticed that the "force" selection starts at 25 lbs. As I haven't started the steel part of the build yet, but based on some known weights for components I'm planning to use, I don't think all the component will weigh 25lbs (11.?? Kgs) Will this be a problem assuming I'm using the PK296A2A SG7.2 motors (probably with a 20 tooth pinion)?

Gerald D Tue 01 July 2008 01:23

David, I think you will be okay. There is a small chance that you will need to add some mass to balance the slide if you use that 9416K54. If you go to a spindle one day, you can remove that mass.

KevinL Wed 03 December 2008 21:02


In the original post to this thread you mention a "wire coil spring" approach. I don't see any more dicussion of that. I've been looking at alternatives to multiple gas springs since I built my Z-slide with about 24" of travel. I'm considering a pair of constant force springs (think heavy duty tape measure return spring). Probably McMaster-Carr part number 9293K24. Is this what you had in mind, or were you thinking more of a garage door spring (torsion) plus some type of cable?

Gerald D Wed 03 December 2008 22:31

Kevin, I have havn't given any more thought to wire coil springs further to the sketches in post #1 right at the top of this thread. At that point I was simply talking a "tower" (the red part) on the y-car, holding the top point of a long conventional extension spring (continuous length extension springs). A "garage door spring (torsion) plus some type of cable" would be a more compact & elegant solution. (we can buy those parts as spares)

smreish Thu 11 December 2008 18:57

My new modified long stroke z is counterbalanced by THIS. It can be configured upto 1.8 M @ 35# (96" @ 35#) or ordered in one of 4 configurations of resistance.

I'll let you know how it turns out.

domino11 Thu 11 December 2008 19:21

That looks interesting, are they expensive? I know myself as well as a few more here are eagerly awaiting your results with the long Z!

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