Hi Everyone.

I spent some enjoyable time creating an MOC gearing mechanism that produces an exact 7:1 gear ratio and can be modified to create a 13:1 ratio. It's not that practical but shows how I converted an idea into a working model - it's never guaranteed that what you think will work actually works in practice. I think I have inadvertently created a variation of a differential gearing.

 

 

8tooth gear to turntable is already 1:7, so it should be much simplier. But it looks good.

1:13 gearing
Achieved but not very practical, and with all those gears there will be friction losses.

There is  a solution in Yoshinto Isogawa Machines & Mechanisms book on page 166.

It uses a Z56 turntable and two Z28 turntables.
It does not use a differential.
Other gears used are 5 x 8T / 3 x 24T and 1 x 20 double bevel gear.

The book also shows a 1:17 gear set up using a Z28 diff.

Edited February 5, 20205 yr by Doug72

Now built a 1:13 and a 1:17 gearbox as shown in the Yoshinto Isogawa Machines & Mechanisms book.
Both give the  exact ratios.

I have no idea how these gear ratios are obtained !!

The 1:13 uses planetary gear with a Z56 turntable which gives 1:4 ratio - so 13 / 4 = 3.25
So the other external gears must counter rotate the planetary gears.
Note the Z28 turntables must be used, won't work with the new 28T gear which has a cross hole. 

Edited February 5, 20205 yr by Doug72

I'm having trouble parsing the structure, let alone calculate the ratio, but I suspect the gear next to the big turntable in the 1:13 is a Z28 turntable connected to the carrier of the planetary system, the right crank is connected to the sun, and the left axle is connected to the orbit. The rest of the geartrain locks the carrier and the orbit in a fixed ratio. So the planetary system is not a simple 1:4 ratio which IIRC is what you'd get if the orbit was fixed.

12 minutes ago, pleegwat said:

I'm having trouble parsing the structure, let alone calculate the ratio, but I suspect the gear next to the big turntable in the 1:13 is a Z28 turntable connected to the carrier of the planetary system, the right crank is connected to the sun, and the left axle is connected to the orbit. The rest of the geartrain locks the carrier and the orbit in a fixed ratio. So the planetary system is not a simple 1:4 ratio which IIRC is what you'd get if the orbit was fixed.

Correct I think !

All I know having built them they work.!

The book is a mine of info for practicable solutions for gear ratios, step / step down / exact numbers.

Okay, taking a shot:

For the  left, starting from the left flag along the left side, 24:8:8 and 24:24 means the flag making a full rotation down means the orbit makes three full turns up.

Along the right side, 28:12:28 means the flag making a full rotation down means the carrier also makes a full turn down.

Googling a planetary gear calculator and putting the numbers in, this indeed gives me 13 turns on the sun.

 

For the right, again starting at the left flag, 24:8, 24:8, 28:12:28 gives the flag making a full rotation down causes the differential case to make 9 full turns down. This means the average of the left and right differential inputs must also be 9 full turns down.

The left side is fixed at one full turn down, so the right must make 17 turns down for the average to work out.

Understanding the right-hand side makes it rather easy to adapt it into a 19:1 ratio: Simply invert the direction between the left flag and the left differential input while leaving the other ratios and directions the same.

Edited February 5, 20205 yr by pleegwat

59 minutes ago, pleegwat said:

Understanding the right-hand side makes it rather easy to adapt it into a 19:1 ratio: Simply invert the direction between the left flag and the left differential input while leaving the other ratios and directions the same.

I have turned the diff around and it does indeed give 19 :1 ratio.

Edited February 5, 20205 yr by Doug72

On 2/6/2020 at 8:09 AM, Doug72 said:

Now built a 1:13 and a 1:17 gearbox as shown in the Yoshinto Isogawa Machines & Mechanisms book.
Both give the  exact ratios.

I have no idea how these gear ratios are obtained !!

The 1:13 uses planetary gear with a Z56 turntable which gives 1:4 ratio - so 13 / 4 = 3.25
So the other external gears must counter rotate the planetary gears.
Note the Z28 turntables must be used, won't work with the new 28T gear which has a cross hole. 

 

Hi Doug,

Thanks for that reply. A few people have recommended Isogawa's book so I have now ordered it from Amazon! Look forward to learning from it.

Thanks again,

Rob

 

 

 

1 hour ago, TechnicBrickPower said:

 

Hi Doug,

Thanks for that reply. A few people have recommended Isogawa's book so I have now ordered it from Amazon! Look forward to learning from it.

Thanks again,

Rob

There’s another book on “Cars and Contraptions”which is also very useful for ideas etc.

Thanks again Doug. I have ordered that one also - looks like a great book with many ideas and examples. Sometimes it's easier to start with someone else's solution and modify that from there.