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Need help to get size

j king

Stainless
Joined
Oct 27, 2003
Location
ohio
I am changing the drive on my lathe.It had a varible speed lever and a surface speed dial.I am installing an AC drive and want to conect the pot switch to the hand lever that originally adjusted the speed.
This is what I need.The lever moves 80 degrees.The pot switch looks to rotate 3/4 of a turn.I would like to make a large and small gear to conect the 2 and get the movement to complete the range of motion of both pieces.I cant seem to see a segment of an arcs travel in relation to the pot switch.The small gear will be 1/2" dia.If you need more infro holler.Thanks. Jim

[ 09-09-2006, 09:56 PM: Message edited by: j king ]
 
Jim

Are you saying that you want the lever that moves 80 deg full travel to also move the pot it's full travel of 3/4 turn or 270 deg ?

If so you need the 270deg/80deg ratio with the larger gear on the lever and the smaller gear on the pot...

Do you have room for a large gear on the lever? and then mount the pot with it's smaller gear to mesh with it? If so we can figger the actual gear selection.

You say the small gear is 1/2 in Dia...

How many teeth on it?
 
It sounds as if you are at the early stages of your conversion so perhaps some changes could be considered.....do you have the room to mount up miniature cog belts? Use sheaves in the ratio needed. This keeps the pot going in same direction, if that's a consideration. Berg sells them small enough.
 
Gary. You got it.I knew the ratio but am stuck at the rotation amount.Yes I have room and that is what I want.I dont have the gear made.I was using this size as a starting point.It can be smaller.

Bluchip.Yes getting started.I have the motor in and all I need to do is figure this out and wire start stop to existing levers and that is no problem.
I thought it was 3 to 1 ratio but that didnt work .I was really wanting to use gears.Didnt think about a belt tho.It needs to be compact.
I thought the pot worked either direction by switching wires.I was assuming this.I am probably wrong then.
 
Before you execute this conversion consider the implications. It's true you can electrically vary the motor speed over a wide range with good speed regulation.

It's also true that a motor is a constant torque machine. If the motor develops 2 HP at full speed it will develop 1 HP at 1/2 speed 1/2 HP at 1/4 speed and so on. There is no adjustable mechanical advantage between the motor shaft and the spindle hence the spindle is driven by the torque of the connected motor whose HP output varies with shaft RPM.

Constant torque per se is not an inherent defect of AC drives DC drives or any other attribute of electric motors. They behave as their physics dictate. If we elect to use an all-electric drive we should do so with an understanding of the underlying physics of electric motors.

Your old existing variable speed pulley arrangement is a constant HP machine. As you reduce the spindle speed, motor speed stays constant. The spindle torque increases in rough proportion to speed reduction thanks to the changing mechanical advantage of the variable pitch pulley and belt system. Adjusting the lever changes the pulley ratio and with it the mechanical advantage so the spindle HP stays more or less constant.

You have several options. One is a install a 5 times oversized motor and drive intended to provide sufficient torque at lowest RPM - this is a common expedient in CNC machines. Another is to install a multi-step pulley drive between motor and spindle. The final option is to live with the reduced spindle HP at lower RPMs whuch - trust me - will be ornerous to live with when the spindle speed range has to vary over 7 to 1. The lightest stock removal cut on a large diameter at minimum RPM may stall the spindle drive.

There is yet another option which is to run a specially designed DC motor in field weakening mode. Good designs of field weakening drives can acheive a constant HP over a 5:1 speed range; a range sufficient for your variable speed lathe. These motors can be very expensive or if scrounged on the used market very difficult to find. The electrical drive while not complicated will require some knowledge of how field weakening drives work before a successful design can be executed.

So rip out the variable speed drive with due caution. My suggestion would be to restore the old mechanical drive to like new performance and use a VFD drive to run the existing motor from single phase.
 
Hey Forest.This was a dc varispeed Monarch.The speed control was changed by the dc motor not a sheeve setup.It only has 3 gear ranges.The motor on it was 460 volts only so I had no choice(in practical terms)than to install the ac drive and motor.It will work fine.Maybe not the deep low rpm grunt but should do fine with a 10 hp motor.This will be run off 3 phase. Take care, Jim
 
Jim, you should be able to get all the speed range you need with the 80 degree travel. For my VFD I set the POT at 60hz in the center of the range (pointer pointing straight up).
 
Sorry. Those little details can really trip up a line of advise. I concluded it was a LeBlond or similar vari-speed drive.

A 10 HP brute force drive works very well with a Monarch AC retrofit.
 
If the direction of rotation needs to be the same and you use gears put an idler gear in. This will sometimes even make the design of the gearing easier. An idler will not change the ratio at all. Also gives you more flexibility with the center distance. A wrap of polyester cord around a wheel like the old radio tuners used to move the needle is very reliable in the right enviroment. Not real familiar with your application.
 
270/80=3.375 3 3/8 is the ratio right? So if the pitch dia. of the smaller gear is .500,1/2 of 3 3/8 is 1.687 for the pitch dia. of the driven gear. Sounds about right, a little over 3 to 1.
 
Pepo .That is right .I bought some plastic gears and laid them out and it didnt come close.The small gear didnt rotate near enough.Maybe I messed up on the bench testing this.I will need to get another set of hands and try again but it sure seemed way off.

Ralph.I want to go to 100Hz or 110.Need more travel. Thanks guys.
 
Jim,

Since you didnt say much about the small gear, lets make something up..

Small gear must rotate 270 deg
Small gear has 48 teeth ( you get this by reaching up in the sky and picking a number...anynumber)
270 deg of rotation is 36 teeth

Solve for larger gear.....

Large gear must move 80 deg and 36 teeth

Since 80 deg is 36 teeth, then 360 deg is 162 teeth

We need to know what pitch you need to determine the gears diameters. And other gears of the 162/48 ratio can be used...such as 81/24
 
forget gearing down the lever mechanicaly. use a higher resistive pot but only turn it the first 80deg. agood pot (not wire wound)even with the reduced movment will still give step free smooth presicion. re adjust the vfd parameters if need be to still get desired top rpm if you can not get correct value pot
 
adama

I think that would not be a good idea. Here's why...It would mean a set rotation of the lever would make a larger (more than 3 times) incremental move in the output of the pot. Think of it this way... replace the gas pedal on the Corvette with one 1/3 as long or with 3 times the motion to the engine....press it a little and your at 80 mph instead of the 40 you wanted...press it justa little more and your at 160 mph...
 
gary e

doing this electricaly means no gear slop. it will still be as sensitive as hell with gears as electrics. the part that scares me the most would be acedentaly catching the lever whilst turning some nice big of center lump. iam not saying this is a good idea just that it is easier to due it electronicaly than mechanicaly to get the same result. just because a pot has 270 deg of movment does not mean you have to use it all and this is what jking is trying to do ie full speed range over 80deg of movment
 








 
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