Replacement screw lengths and spring constants

[CarlBrannen]

I've got a Shengshou 7x7x7. I wondered what would happen if I changed the springs. The SpeedCubeShop sells spring kits for Dayan and Alpha (Type A), for only $1.25 each so naturally I ordered a set to see what would happen.

So I've got three different sets of screws to mess around with and three different springs.

--------------------- Screws -------------------------
All three systems use the same diameter and screw thread. My calipers give an outer thread diameter of 0.114" and a screw pitch of .020" so I conclude that these are metric M3x0.5 http://www.engineersedge.com/iso_phi...head_screw.htm

The lengths of the screws, and the length of threads differ:
Shengshou 7x7x7: 0.860" (22mm), threaded 0.360" (9mm)
Dayan: 0.789" (20mm) threaded 0.323" (8mm)
Alpha (A) 0.767" (19.5mm) threaded 0.288" (7mm)

In each of these the threading doesn't begin until around 2mm from the end. I'm not sure that that's a necessary feature of the design. My suspicion is that these are cheaper than the standard Philips head M3x0.5 which sell for around $12 per 100 at McMaster Carr (click "metric" and then your choice of head"): http://www.mcmaster.com/#machine-scr...teners/=i3jawb

--------------------- Springs -------------------------
The springs are "closed ends". The springs have outer diameter, wire diameter and lengths of:

Shengshou 7x7x7: outer: 0.178" (4.5mm), wire: 0.026" (0.66mm), length: 0.614" (15.6mm)
Dayan: outer: 0.181" (4.6mm), wire: 0.024" (0.61mm), length: 0.395" (10mm)
Alpha (A): 0.179" (4.5mm), wire: 0.025" (0.63mm), length: 0.422" (10.7mm)

McMaster Carr has similar springs such as the 0.18"x0.024"x0.5" item# 9657K267. I didn't measure the "rate" (physicists call it the "spring constant") which McMaster Carr gives in pounds per inch for the inch type springs and, amusingly, pounds per mm for the metric springs. Anyway, I didn't measure the rate for these three springs, but if there's any real interest I could do it easily enough. Now that I think of it, I can also measure the frictional coefficient between different plastics.

McMaster Carr's metric springs have a minimum diameter of 5.5mm which would fit in my Guhong version 1 (which has an inner plastic sleeve diameter of 6mm) but it won't go in my Shengshou 7x7x7 with its sleeve diameter of 5mm. These metric springs are closed and "ground flat" and might make a nice upgrade for a Guhong (and probably other 3x3x3s), as the ground flat would presumably give more consistent resistance. No way it would make any difference for me.

--------------------- Design -------------------------
In choosing a spring / screw there are three considerations (in addition to the mechanical size requirements):
(1) Spring length
(2) Screw length
(3) Spring "rate" or "spring constant" or stiffness.
(4) How far the screw is screwed into the spider.

These choices effect the cube's friction, popping, locking, and corner cutting so they are very important. By making changes in these things we can change three things:
(a) How much force is on the spring when the cube is resting.
(b) How much play (separation) there is in the cube when the springs are fully compressed.
(c) How much force is on the spring when the spring is fully compressed.

Maybe a drawing will help. This shows the force on the spring as the cube is pulled apart:
spring.png

--------------------- Example -------------------------
I tried the Dayan and Alpha springs in my Shengshou 7x7x7. The Dayan springs are so short that they are not in compression even when the screws are all the way in. This makes the cube exceedingly loose, but with very low friction. I set it up with very little play so it's impossible to pop.

Next I tried the Alpha springs. They're a little longer and so there is a very slight force when the screws are nearly all the way in. I solved it with no great difficulty, but it's inferior to the standard springs in that it has so little play. If I adjust the springs for more play, the result is a very loose puzzle. This seems to make it easier to lock.

So to try these other springs, I will have to effectively lengthen them by adding washers.

[wytefury]

Well I have something to maybe help you a little with this problem. I kind of thought the same thing as I got my SS 7x7 awhile back. I said to myself, "Well, I changed the springs in my SS 6x6, lets do it to me 7x7 too!" So I uncapped the cube and took out the screws and I noticed immediately, just like you did, that the stock SS 7x7 springs are longer than normal. I was thinking about just leaving it, because honestly the springs in the SS 7x7 arent too bad. But, I just kept trying to think of a way to change the springs...I was just curious if it would help it a lot. I finally thought of something. Here's a picture...(Ill explain below)



What I did was cut 2 C4U springs right beneath the bulge that there is on the springs. And I put them together (now they were the same length as the stock SS 7x7 spring) , putting a washer inbetween the two to prevent possible spring twisting or something (it just didnt seem right to me sticking two springs together like that without a washer). Anyway yeah. It did help. I noticed a change in the performance of the cube. It being faster, smoother, etc. But is it worth it and necessary? Not really. In my honest opinion. Sure it helps, and if you're very serious, sure try it. But for others reading this post, its definitely not a needed change to the SS 7x7. Also, I noticed you said something about "filling" each center piece with enough washers to make the springs long enough. I think you would need A LOT of washers for that. I don't know it it would be worth it.

So to do what I did you will need...
12x Any type of spring (I used C4U, but I am sure Dayan would work also) (I strongly suggest not using Alpha though, as they are as stiff or stiffer than the stock springs)
12x washers (sorry, I forgot to put both washers in the picture, but its a given it would need two, one inbetween the two springs and one underneath them.)

Hope this kind of helps you (not sure if this was your exact problem/question but yeah). Good luck!

[CarlBrannen]

Finding just the right spring in a catalog looks to be a tough chore. I'm thinking that the right way to experiment with new springs is to buy some long springs that are to be cut to length.

Rather than filling up the space with washers, you can use little cylinders. I'd probably turn them on the lathe. And the best choice of material might be plastic. With the Shengshou 7x7x7 what you'd be doing would basically be filling in some of that deep well in the center pieces.

This reminds me of another mod to a Shengshou 7x7x7 I'm going to try; gluing the two center pieces together. There's a loose flat square and the outer square with the deep well for the spring. It seems to me that the only reason they didn't make it a single piece was because it would have been difficult to mold. In fact, I'm going to do it right now... I think the only difference is that it will be slightly more difficult to assemble, given the way I do it.

[littlewing1208]

@CarlBrannen,

Did you even find a good McMaster PN for springs for various cubes that have approximately the correct spring rate? My SS 4x4 now that it is modded it way too loose and the screw bottoms out on the center piece before it is tensioned where I want it. Couple this with the fact that the SS cubes have those incredibly skinny 'sidewall' washers (about 3mm ID and about 4mm OD whereas Dayan washers are 3mm ID and 5.7mm OD). So I basically would like to get a spring with the same spring rate but longer.

Any advice?