Series planetary gear speed reducer

[zain1]

Hello,

I have to design a series planetary gear speed reducer.
I have to ask first that what is SERIES planetary gear? Can anyone give some detail about this.

I have to design a gearbox with three stages with a motor power of 45 KW. Input speed of Motor is 2800 rpm, Max outer dia of gearbox is 100 mm. the gear ratio is 4:1 which is same in all three stages. I am doing this for the first time so please anyone can tell me in detail about how I can proceed...
This gearbox is basically for an oil rig. which have limitations of size. the gearbox is of 3 stages because of this small size.
the gear ratio is 4:1 for all three stages that means I have to keep the gears of same size in all three stages... This is because of the small size to divide the forces and torques
Thanks in advance.
Regards,
Zain

 

[Unrest]

Just put 3 planetary gearsets in series. There would be different configurations of which parts to lock together, but you can just work it out to get the desired effect.

I suspect you can have them all sharing a common ring gear, and have the planet carrier of one stage connected to the sun gear of the next.

 

[zain1]

Hello,
Thanks for the reply.
Can you kindly tell me about it in detail what should I do with forces, torque, gear ratios etc

 

[jambaugh]

In the idealized frictionless case the power throughput is constant.

Since power is force* speed = torque times angular speed (e.g. radians per second). You'll see the torques go up as the rotational speeds go down.
Three stages of 4:1 gives 4x4x4 :1 = 64 to 1. The output speed will be 1/64th the input and the output torque will be 64 times the input.

Thus the output power = input power.

 

[DickL]

I've not run any numbers on this, but it seems to me that will be an awfully large output torque for a gear unit that is only 100 mm in diameter (and I assume that includes the gearbox).

 

[kriakkk]

hey buddy, if the speed redues to 64th of 2800 gives 43.75 rpm. Its too less, may be, for oil rig. I didnt understand your requirement. however u can increse the gear width for the gear to take more load.

if u take pinion 20mm and internal gear(annular gear) of 80 mm you can achieve of 1/4th of reduction probably vth a width of sme 40-50mm try out.

 

[zain1]

the overall ratio is 4 and the output speed is 700 rpm. but I have to divide it in 3 stages because of the torque is so high for 100 mm outer dia. the gear ratio will not change in three stages it will be same in 1st,2nd and 3rd stage. I think this can be achieve by using pinion and gears of same sizes in all three stages. so that the speed is not reduce in between the stages. but the forces are divided.

 

[Unrest]

the overall ratio is 4 and the output speed is 700 rpm. but I have to divide it in 3 stages because of the torque is so high for 100 mm outer dia. the gear ratio will not change in three stages it will be same in 1st,2nd and 3rd stage. I think this can be achieve by using pinion and gears of same sizes in all three stages. so that the speed is not reduce in between the stages. but the forces are divided.

I'm wondering why you have the constraint of 3 stages. How do you know 1 or 2 won't work?

Anyway, it's a lot easier now you don't have such high torques! Just make each stage's ratio the cube root of 4.

I don't understand the last part of what you said. It sounds more like a parallel arrangement.

If you want to get a feel for the geometry of this, you can find series planetary gearboxes in cordless screwdrivers.

 

[zain1]

there is not a constraint of 3 stages. may be 2 is ok.

it's a lot easier now you don't have such high torques! Just make each stage's ratio the cube root of 4.

I don't understand this... why cube root of 4...?

 

[256bits]

I don't understand this... why cube root of 4...?

because basically...

speed reduction 1st stage x 2nd stage x 3rd stage = 4
cube root 4 x cube root 4 x cube root 4 = 4

 

[kriakkk]

wat Unrest said is rite. y 3 stages u cn do it even in 1 stage or 2, the prob here is the high torques. So i think u have to increase the gear width and take the better material.

U cn go for 3 stages of cube root of 4 also. diameters cn be same in al 3 stages, wat abt width of gear, it cnt be same, the load on tooth increase as rpm goes down. if u have no constraint on width u cn even do it in 1 stage.

if u don't mind cn u tel the details of the oil rig fr vch u rake designing dis gear box.

 

[zain1]

I don't have the details of oil rig... I only have the specs of motor and the requirements...
can you please tell me how I calculate the torques and forces on the shaft to start it...
waiting for the reply

 

[kriakkk]

Power = 2*pi*N*T/60 u might have known this formula N in rpm T is torque in Nm.
U cn get torque here.

Arbitrarily decide the dia of pinion nd annular gear. there is forluma in gears (read it frm book) u cn use that to find the width of gear. u need to knw the unltimate stress of the material for that formula. I dnt remember the formula rite knw, i too need to go thru buk.

Is dis helpful? read a book on gears, solve a few problems give nat the end of chapter u can design similar way. Its diffi to explain here completely.

 

[Unrest]

Yea Kriakkk has the general process. Or if you don't have the special gear torque formula, start with the formula for planetary gear ratios (Wikipedia). From the ratios you can know the gear diameters and then the forces on the teeth and their stresses.

 

[zain1]

Thanks all for the replies.
One more thing I want to ask is about the shaft. As I want to design a series planetary gearset. So how I calculate the torsion on shaft at diff stages.

And one more thing about the series planetary gearset. How it works and different from others...

 

[Unrest]

So how I calculate the torsion on shaft at diff stages.

Look at just one stage and ignore the others. Do the analysis on that to find the torques. If you know the torque at the input then the gear ratio tells you the torque in the output shaft regardless of whether it's connected to another gearset or a load or whatever.