Running something up the flagpole to get opinions please.

OK, you all know that I am a fairly recent "lathe-boy" and that of course "if you only have a hammer everything looks like a nail". :)
Paul recently sent me a Longstaff 2WD unit from an English trike so I could have a play with it.
Apparently people are discarding them because they have a very limited 6-speed set of gearing .

I compared it to one that I had made that can accept any modern 9-11 speed cassette or custom cassette.
My version was slightly longer but also 100gms lighter. :)
We then had a discussion about how such a 2WD unit could be made with the most minimal set of tools.

So I had a bash at "how" I would do that IF I did not have a lathe.

This what I came up with (apologies for the blurred image):

image.png


On the left is an "off the shelf" screw on freewheel, on the right is an "off the shelf" screw-in cassette carrier freewheel.
The pair are available from evilBay for <£20 combined.
Joining them together is 2-sizes of tubing that is welded to the "outer race" components of each freewheel.
The RH screw in freewheel has a 10mm hex machined in the end, but we cannot use it as a drive element because the axle section of the freewheel can only be 12mm OD and that is too small to create a 10mm hex from.
My proposal was to cut most of that threaded section of the RH freewheel off and weld a 6mm plate over the exposed end.
A square hole is made in the plate by drilling and filing to create a drive-plate for the squared-off end of a 12mm axle (again, just marking out and filing).
In the LH end we insert a standard BB cup with a plate welded over it and....guess what? That's right another 8mm square hole formed in it by drilling and filing.

In this way we have removed the need for a lathe and we have created a ratchet driven 2WD unit that drives 2 independent half-shafts.
As the drive unit sit in the middle it would be necessary to have inboard support bearings on the half-shafts, but that is probably easy to do.

So that’s it in a nutshell. That would probably be as small and as lightweight as I could make one without resort to proper tooling.

Making the drive dogs square means that anyone with access to a welder a set of files and a drill can form the holes/squared ends of the shafts and make this unit. The biggest square you can make out of a 12mm rod is an 8.64mm square. I looked up the shear strength of an 8mm pin and it is in the many hundreds of foot-pounds, so I think it would suffice as a drive-pin.

The bigger risk (i think) would be that such a small square pin manages to "round-out" the square hole it is inserted into, but it is always possible to quench harden the plate with the hole in it.

Anyway, that’s a pretty cheap 2WD unit IMHO. :cool:

Your thoughts/observations are much anticipated.
 
The bigger risk (i think) would be that such a small square pin manages to "round-out" the square hole it is inserted into
I'm also having a hard time understanding, but I do think that an 8mm square pin is a pretty minimal bearing surface for an anti-rotational fixture.
I think it would quickly round out, as you say, and if hardened, likely torque off just as quick.
That, plus the fact that a hand-filed hole is probably not going to give a very close fit, for most folks, means that only the corners of that pin are likely going to be taking the load.
If it was a precision press fit, it might hold up better, but still not great, I think.
 
OK, thanks for the feedback. I will try to expand/clarify. I can't solve the lack of bearing surface without resorting to a lathe and re-making the innards of the RH unit and most folks don't have a lathe. :(
 
A second reading and I see it now. A 10mm o/d nut could be welded onto the 12mm shaft for the drive on the right. It's going to be a weak point though.
 
A second reading and I see it now. A 10mm o/d nut could be welded onto the 12mm shaft for the drive on the right. It's going to be a weak point though.
But, the users will want to be able to slip the shaft out to make changes to the cassette. Whatever I do with that RH side the connection may be perilously small. An 8mm bolt has a shear strength of 6,475 foot-pounds, which sounds a lot, but may not be. We have all seen/had bolt-heads round off and become useless.
But..
This load will be a twisting shear strength and my mate ChatGPT says the actual twist-shear is very low at about <60 foot pounds.
Here's the convo..
what is the twist-shear strength of an 8mm square pin of silver steel?

ChatGPT said:​


The twist-shear strength of an 8 mm square pin made of silver steel (also known as drill rod) depends on both the material properties of the silver steel and the cross-sectional area of the pin.

Step-by-Step Calculation​

  1. Material Properties:
    • Silver steel typically has a tensile strength of approximately 700–800 MPa when hardened. For non-hardened silver steel, it might be closer to 500–600 MPa.
    • The shear strength is typically around 60% of the tensile strength for steel. For hardened silver steel, it would be around 0.6×7000.6 \times 7000.6×700 to 0.6×8000.6 \times 8000.6×800 MPa, approximately 420–480 MPa.
  2. Cross-Sectional Area of the Pin:
    • Since it’s a square cross-section, area A=8 mm×8 mm=64 mm2A = 8 \, \text{mm} \times 8 \, \text{mm} = 64 \, \text{mm}^2A=8mm×8mm=64mm2.
  3. Twist-Shear Force:
    • The torsional shear strength is determined by the cross-sectional shape and area, as well as the applied shear stress. This is given by:τ=T×rJ\tau = \frac{T \times r}{J}τ=JT×r
    • where:
      • TTT = applied torque,
      • rrr = radius to the outer edge of the square,
      • JJJ = polar moment of inertia for a square section.
Would you like me to complete the calculation with typical values or use specific values you have for the shear strength of your material?

4o

You said:​

yes please

ChatGPT said:​


To find the twist-shear strength of your 8 mm square pin of silver steel, let’s go through the steps using typical values for silver steel.

Given Data​

  1. Material Shear Strength (τ\tauτ):
    • For hardened silver steel, approximate shear strength is around 420–480 MPa.
    • I'll use an average value of τ=450\tau = 450τ=450 MPa.
  2. Dimensions:
    • Cross-sectional area A=8 mm×8 mm=64 mm2A = 8 \, \text{mm} \times 8 \, \text{mm} = 64 \, \text{mm}^2A=8mm×8mm=64mm2.
    • Radius to the outer edge of the square r=82=4 mmr = \frac{8}{2} = 4 \, \text{mm}r=28=4mm.
  3. Polar Moment of Inertia (JJJ):
    • For a square cross-section of side length aaa, J=a46J = \frac{a^4}{6}J=6a4.
    • For a=8 mma = 8 \, \text{mm}a=8mm, J=846=682.67 mm4J = \frac{8^4}{6} = 682.67 \, \text{mm}^4J=684=682.67mm4.

Calculation: Torsional Shear Strength (Torque, TTT)​

The maximum torque TTT the pin can handle is given by:
T=τ⋅JrT = \frac{\tau \cdot J}{r}T=rτ⋅J
Plugging in the values:
T=450×682.674 N\cdotpmmT = \frac{450 \times 682.67}{4} \, \text{N·mm}T=4450×682.67N\cdotpmm
Let me calculate that for you.
The maximum torque that the 8 mm square pin of silver steel can handle, based on a shear strength of 450 MPa, is approximately 76,800 N·mm (or 76.8 N·m).
This is the twist-shear strength, indicating the torque at which the pin would begin to fail in shear.

4o

You said:​

what is that in foot pounds

ChatGPT said:​


The twist-shear strength of the 8 mm square pin of silver steel is approximately 56.65 foot-pounds.

So I shall have to think of another way to do this.
 
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Dan

If you want it easier understood why not lay the parts out in a straight line on some A4 paper in the order of assembly and label them , then take a picture and post that ?
You will then be able to talk people through the build and your decisions to get this far ?

The chosen cassette free wheel is limiting the right side drive shaft options , however they are available on 15mm through hubs [ I have one {although I am not sure the bit you show can be bought separately ]] which are more costly than your option ?

What about thinking outside the box [ this could lead to failure ! ] can you buy M12 hex rod ? if so and drive shaft was made out of that then the coupling part is already preformed ?

Maybe the free wheel end could have sockets welded to it for the interface ? [ ok it will be a bit heavier ] however there is some meat that can be shaved off them ?

So you say what about the bearings ? could some precision tubing with a O/D of 15mm be persuaded to fit the M12 hex rod at the points where you need the bearings ?

Sadly whilst DannyC in a moment of weakness may succumb to building one , it really needs someone to step up to the mark and deploy/ride it to discover if it actually works.

Sadly my quad has a completely different drive train

Paul
 
Danny
If a 10mm across the flats nut was welded to the end of the 12mm rod it would fit snugly into the 10mm hex with a perfect fit. The thing would just slide in or out.

Paul
12mm across the flats is just under 14mm across the corners. you'd need very thin tubing to end up with 15mm od.

Beware of AI. I had chat GPT work out the maximum deviation an airgun pellet would deviate if it hit a large raindrop as I do a lot of target shooting. It came up with several hundred degrees deviation as the answer. When I pointed out the maximum possible was 180 it sagely agreed with me without even having another go at it. AI is very unintelligent.
 
Danny
If a 10mm across the flats nut was welded to the end of the 12mm rod it would fit snugly into the 10mm hex with a perfect fit. The thing would just slide in or out.

Paul
12mm across the flats is just under 14mm across the corners. you'd need very thin tubing to end up with 15mm od.

Beware of AI. I had chat GPT work out the maximum deviation an airgun pellet would deviate if it hit a large raindrop as I do a lot of target shooting. It came up with several hundred degrees deviation as the answer. When I pointed out the maximum possible was 180 it sagely agreed with me without even having another go at it. AI is very unintelligent.
Yup, I have had similar experiences with AI. It always gracefully apologises and has another stab at it. :)
 
I am not disheartened, I just have limited "parts" at my disposal to mess with.
I have 2 cassette capable freewheel units one is the type in this thread with that big 12TPI threaded section that screws into "something".
Another is the sort that has the "TORX"-like interface where the cassette carrier and ratchet are one half and the other interface is on the hub, and it is the 10mm axle and its bearings that locks them all together.

If I "ditch" the "no specialist tooling" requirement it makes it all easier, but I was trying to make this something a bloke in a shed could do for a scout team that wanted to build a pedal-car racer.

If this sort of thing doesn't get done that sport is likely to die out. There are only 2 (maybe 3) actual builders so once they get too old the sport might be "kaput" and that would be a shame. :(
 
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Paul
12mm across the flats is just under 14mm across the corners. you'd need very thin tubing to end up with 15mm od.
So the flats are only needed at the end it interfaces with the free wheels so surely the flats can be flap disked down till some M15 x 1.5mm wall tubing fits over them ?

That of course moves the problem to ' how do we attach the wheel ' :rolleyes: ;)
 
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If I "ditch" the "no specialist tooling" requirement it makes it all easier, but I was trying to make this something a bloke in a shed could do for a scout team that wanted to build a pedal-car racer.
Specialist tooling probably isn't all that uncommon, nowadays - There's lots of hobby machinists out there, with little Chinese lathes in their sheds.
I mean, you'd realistically still need to weld, and some people would consider a welder to be "specialist equipment", if all they had was a hacksaw, a cordless drill, and a wrench set.
Just the fact that you'd dream of building something like this, probably means you're already in the realm of the "specialist tooling" hierarchy ;)
 
I didn't use a lathe for the back axle of the Church Pod. Just tube stepped down one inside the other with12mm at the end for the axle. The axle is the fattest I could pass through a BB cartridge type cap which mounts both the diff freewheels and the central drive one by welding to a split clamp. It uses a screw on rather than a cassette but it's simple enough to replicate. In that instance the diff freewheels are by the wheel not central. Robbed shamelessly from the Rhoades. Admittedly it still needs finishing and testing. If those M8 drive bolts want to rise out of mesh I may have to stiffen the central part of the disc.

Pcfmssz.jpg
 
When those M8 drive bolts rise out of mesh I will have to stiffen the central part of the disc.
Edited for clarity....Just my little joke of course.
Any audio resemblance to an old-time football "Rattle" will be purely coincidental. :)
 
Thanks for the idea and video, it would be really useful to make a rear axle "pseudo differential" with freewheels and without a lathe.
Although there are still in my area places where you can get small parts machined that ends up being pricy if you only need a few units.
for locking the axle, would it make any sense to cut off & reuse a bottom bracket axle and the center of a steel pedal ?
the square / centering / tight fit is well machined but I guess that pushes the difficulty to aligning everything and welding perfectly aligned.
 
As a progression from my earlier version and the demand that it be a "lathe-boy" free endeavour, here's another variant I will be putting together in the coming week(s).

Option 2 – Screw on freewheel Version.​

This version is likely to be more robust because none of the parts are really being taken out of their normal operating mode.

The cast in order of appearance are (left to right) numbered 1..5.

  1. A BB shell cup.
  2. A cheap freewheel with a 58mm diameter outer race.
  3. A slice of 60mm OD tube.
  4. Another BB Shell cup.
  5. A screw on 8,9,10 speed freewheel.
image.png


Not shown Because I don’t have them) are the sawn-off ends of 2 “X” mm box spanners (to suit Hex-ends of the axle shafts). I would suggest 13mm box spanners available at c.£2.00 online. These get welded to the BB cups.

IF you have a lathe you can make 2 slices of threaded bar with the BB cup thread on and cut the hex-keyways directly in them of course.

The idea is that the two freewheels (outer races) are joined together by welding the sleeve (3) to the back of part (5) first and then to the front of part (2).

I don’t think welding will be easy, but I think it is doable. If it proves too tricky then I would put a smaller section of tube on each half, weld them in place and then bring the halves together and slip a liner inside the pair for alignment and weld the resultant seam in several places.

It can/will be done. If I do the first one, no-one can say “it can’t be done”. 😊

Operation.​

Left hand half shaft inserts into box-spanner on LH freewheel cup (box spanner bit can be welded on the inside face maybe?).

RH half-shaft inserts thru the RH freewheel and into the box spanner section on the cup of the RH freewheel.

Drive input is to both freewheel’ s linked (linked by the tube (part #3) outer races, and the independent output is from each freewheels independent inner race.

Hopefully the end result then looks like this, and it will prove that anyone can make their own cheap 2WD units.
image.png
 
The issue with welding them together will be to avoid melting all the grease in the mechanisms and the multispeed also will have plastic spacers between the cogs. A bigger tube will put more distance from both of those problem areas for welding heat to dissipate. The multispeed spacers in particular will melt and warp with minimal coaxing. You may need to weld in a location you can do just a spot and then immediately quench it to keep that heat spreading
 
Clever to use box spanners as axle connectors.
I am wondering how are you going to weld and spare the bearings can you take the gears out ?
Or as an alternative for the people who are not stick welding Gandalfs like me, would you think 6 or so bolts between the two gears with part 3 in the middle as spacer could work ? That would only allow the chain to go on the cassette though.
Good luck
 
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