I in designing a single speed reduction gearbox

[Eddie714]

Hello, i am a junior mechanical engineering student and i was given the task of designing (on paper) a gearbox.

I chose to design a worm gear single speed reduction gearbox. I know the basics of what composes it which are the worm, worm gear, input and output shaft bearings and the housing the gears are in, as well as a proper power source which will allow my gearbox to function. I found a gearbox with specifications online, but i need to do some sort of calculations that justify why i can chose them.

For instance, say i gearbox with a 60:1 gear ratio, i am supposed to justify why i this ratio is applicable.

i am also supposed to justify the materials that i am using through some sort of calculations but i am not sure how to.

If someone could set me in the right direction, as to what analysis i should do and with what equations. I would greatly appreciate it

 

[billy_joule]

You haven't provided any info on why the gearbox is needed and what it needs to do. What specifications were you given? Power transfer? Torque load? Rpm in? Rpm out? Temperature? Load type? Shaft layout? Efficiency? Price? Lifespan? Etc etc

 

[Tom.G]

A gear train changes speed and, inversely, torque. Presumably whatever motive source you have doesn't match the requirements of the load for at least one of those... otherwise you don't need any gears (except perhaps to change direction.)

The gear must be strong enough that the torque on each gear will not break off any of the teeth. This will determine the minimum tooth strength (i.e. material strength and tooth cross section.)

The bearings and the gearbox casing need to withstand both the axial an radial loads on the gears/shafts. If the casing deflects too much, the gears can either jam, slip out of engagement, or move apart just enough to exceed the tooth strength, thereby breaking off the teeth (stripping the gears.)

 

[Eddie714]

You haven't provided any info on why the gearbox is needed and what it needs to do. What specifications were you given? Power transfer? Torque load? Rpm in? Rpm out? Temperature? Load type? Shaft layout? Efficiency? Price? Lifespan? Etc etc

The thing is we are supposed to design it from scratch basically, but i found this http://www.mcmaster.com/#gear-boxes/=1220u8j which has everything i need. My question is, for instance what can i apply to say, the input shaft to be able to obtain a minimum size of shaft diameter to support an input torque of say 21 ft-lbs(i don't think its there but i calculated it)

 

[Eddie714]

A gear train changes speed and, inversely, torque. Presumably whatever motive source you have doesn't match the requirements of the load for at least one of those... otherwise you don't need any gears (except perhaps to change direction.)

The gear must be strong enough that the torque on each gear will not break off any of the teeth. This will determine the minimum tooth strength (i.e. material strength and tooth cross section.)

The bearings and the gearbox casing need to withstand both the axial an radial loads on the gears/shafts. If the casing deflects too much, the gears can either jam, slip out of engagement, or move apart just enough to exceed the tooth strength, thereby breaking off the teeth (stripping the gears.)

oh i didnt even think about the fact that the bearings and casing need to withstand the loads. Thanks, this increased my understanding of how a gearbox works. I am assuming that the material of the worm gear is the only thing that makes certain any teeth would break?

 

[Tom.G]

Any place a load has to be resisted. Gear teeth are generally the things with the highest force per cross section. Remember whatever force there is on a tooth, there is a resisting force on the tooth that engages it. (Meshes with it.)

 

[Randy Beikmann]

Two things to consider with worm gears. First, they are inefficient and therefore produce a lot of heat, because the power is transferred almost entirely by sliding action between the gears. The other is that they don't transmit "reverse torque" well - they tend to lock up. This is actually used to advantage in torque bias differentials, like Torsens.

If these traits don't interfere with your application, no big deal. But you'll want to account for them.

 

[Eddie714]

Two things to consider with worm gears. First, they are inefficient and therefore produce a lot of heat, because the power is transferred almost entirely by sliding action between the gears. The other is that they don't transmit "reverse torque" well - they tend to lock up. This is actually used to advantage in torque bias differentials, like Torsens.

If these traits don't interfere with your application, no big deal. But you'll want to account for them.

there's really no right or wrong answer to the choice of gears to use for this particular assignment, i found a worm gear that fits the requirements i need for a stair lift for the elderly and i am kinda basing my work off of that. In myreport i need to be able to explain, through calculations, why i used a 60:1 gear ratio, or why i made my shafts of a certain diameter. I am not sure how to approach this.

 

[Randy Beikmann]

there's really no right or wrong answer to the choice of gears to use for this particular assignment, i found a worm gear that fits the requirements i need for a stair lift for the elderly and i am kinda basing my work off of that. In myreport i need to be able to explain, through calculations, why i used a 60:1 gear ratio, or why i made my shafts of a certain diameter. I am not sure how to approach this.

Using a worm gear for a stair lift makes good sense, because they'd want it to lock up when the motor is turned off.

As far as gear reduction needed in your case, you should base it on the anticipated load you are driving, and the capabilities of your motor. Every motor has a usable amount of torque vs. speed (should be able to get a graph), and your assignment should specify speed and torque for the load to be driven. The rest will follow.

 

[Eddie714]

Anyone know how i can justify the usage of a certain shaft diameter in my gearbox?

 

[billy_joule]

start with a force analysis on the shaft.
This may include axial forces ( see spur vs helical ) bending moments (from radial forces due to the pressure angle) torque etc
Set a max design stress then solve for shaft diameter. You may also want to check angle of twist and whirling speed if you have any long shafts.
There's probably some quick rule of thumb solutions out there too. And gear manufacturers have selection guides which often have more practical information than you might find in a textbook.

 

[24Seven]

Anyone know how i can justify the usage of a certain shaft diameter in my gearbox?

Get your hands on a book titled "Machineries Handbook" in it you will find all the formulas and specs with load capabilities for about anything one could imagine for anything to do with machines from screws to shafts to different metals and loads everything and anything your looking for the answer for concerning machine technology. Every Machinist/inventor/mechanical engineer either has a copy or should have a copy. Great resource for technical data and figures you need to work out how you need to design your lift. Your justification is the book with what is known today in the science of anything mechanical, gear ratios and shaft shaft sizes and torque requirements down to the fasteners and nuts and bolts.