pulleys and servos

[krymis]

why is it that all servo systems that i have seen have a gear reduction in the form of pulleys? Why not mount them directly to the ballscrew? Is there just to much friction in the screw to keep up with the rpm of the servo?

[highspeedmazak]

Not sure but I guess it is because the servos Ive seen on here are cheap ones. Not to cut them down but our mazaks are direct drive. From what Ive seen the ones used here are dc brushed and brushed motors with direct drive are not very strong.

[bunalmis]

Servo motor usualy turns at high rpm and produce low torque. Therefore need reduction.

You can direct drive to screew if you find low rpm high power motor.

[thebodger@roger]

Sorry, but you are totally incorrect about DC servo motors. DC servo motors produce maximun torque at zero, or low RPM, in fact most CNC's of higher quality are direct drive. The use of belts or gears is because of either poor low power motors, or encoder counts/per /rev issues, or motor placement issues where it's inconvienient to have a motor protruding too far from the end of ballscrew. A properly sized motor is ideally mounted with a coupling on the end of the ballscrew.

[H500]

If you stall a servo motor, it will produce a lot of torque, but it will burn up in a few seconds.

On the other hand, if you use a pulley reduction system, you can produce the same torque indefinitely without damage.

To put it simply, pulleys allow you to get the most continuous POWER out of your motor into your load.

[Al_The_Man]

Originally Posted by krymis why is it that all servo systems that i have seen have a gear reduction in the form of pulleys? Why not mount them directly to the ballscrew? Is there just to much friction in the screw to keep up with the rpm of the servo?

It all depends on the application, if a low load/ high acceleration is used the servo can be used to drive a BS direct as in a CNC punch and high lead 1"/turn ball screws are often used, or the same arrangement if high load, but low acceleration/deceleration is used.
If high load - high accel/decel is required, then it is both economical on motor and drive if reduction is used, the engineering guide line is to keep the inertia ratio between motor and load to under 10:1.
If reduction is used, the torque is increased by the amount of reduction, and the inertia ratio is decreased by the SQUARE of the reduction.
Servo's generally have a fairly flat torque curve, with maximum torque at zero rpm.
Al.

[highspeedmazak]

So when it comes down to it these sevors requiring a belt drive for instance we have a cheaper cnc at work it is a basic bridgeport with sero drives and it is belt reduction because they are cheaper seros then comparing them to any mazak servo. The cheaper servos need reduction and yes brush motors are not expensive servo systems they are cheaper and need to be reductioned this gives them some torque that they dont have when mounted direct. Dont get me wrong the belt drives are fine if bearinged right, mounted strong and are in perfect line with each other. I am just no fan of brushed motors.

[Al_The_Man]

Originally Posted by highspeedmazak So when it comes down to it these sevors requiring a belt drive for instance we have a cheaper cnc at work it is a basic bridgeport with sero drives and it is belt reduction because they are cheaper seros then comparing them to any mazak servo. The cheaper servos need reduction and yes brush motors are not expensive servo systems they are cheaper and need to be reductioned this gives them some torque that they dont have when mounted direct. Dont get me wrong the belt drives are fine if bearinged right, mounted strong and are in perfect line with each other. I am just no fan of brushed motors.

DC motors are not fitted using reduction because they
do not have enough torque"!
It is to make a system more economical in both motor and drive, also it is application dependent.
AFAIK Mazak do not make servo motors, historically they have used Gettys DC, Gettys knock-offs with both Fanuc & Mitsubishi systems, later AC servo's with both Fanuc & Mitsubishi branded AC servo's.
The majority of servo motors have maximum torque at zero speed, whether AC or DC, therefore it would follow that if a certain degree of torque was called for, a suitable sized motor would accomplish this, regardless of make.
One of the principle differences between AC & DC is an AC features a much lower inertia rotor.
Al.

[highspeedmazak]

Originally Posted by Al_The_Man DC motors are not fitted using reduction because they do not have enough torque"! It is to make a system more economical in both motor and drive, also it is application dependent. AFAIK Mazak do not make servo motors, historically they have used Gettys DC, Gettys knock-offs with both Fanuc & Mitsubishi systems, later AC servo's with both Fanuc & Mitsubishi branded AC servo's. The majority of servo motors have maximum torque at zero speed, whether AC or DC, therefore it would follow that if a certain degree of torque was called for, a suitable sized motor would accomplish this, regardless of make. One of the principle differences between AC & DC is an AC features a much lower inertia rotor. Al.

Ok you have summed it up but just to ask about:

(DC motors are not fitted using reduction because they
do not have enough torque"!
It is to make a system more economical in both motor and drive.)

Wouldn't it seem to be more economical for both motor and drive to be mounted direct instead of haveing to build a reduction, maybe what you are trying to tell me is they are not very acurite direct mounted? Is this what you are saying? It just seems to me that direct mounted would be alot cheaper to build and alot less more reliable with less service to deal with? Set me straight on this I am not sure anymore. But yes you are right all machines in the old day had steppers first from what I have herd.

[Al_The_Man]

Originally Posted by highspeedmazak Wouldn't it seem to be more economical for both motor and drive to be mounted direct instead of haveing to build a reduction, maybe what you are trying to tell me is they are not very acurite direct mounted? Is this what you are saying? It just seems to me that direct mounted would be alot cheaper to build and alot less more reliable with less service to deal with? Set me straight on this I am not sure anymore. But yes you are right all machines in the old day had steppers first from what I have herd.

Direct on to a precision ground ball screw is about as accurate as you can get, although there are ways to reduce with minimum backlash.
There have been many CNC machines in the past that were direct drive on the B.S., Mazak included, and most were not small machines, by any means.
But when you have to whistle a 300~400lb multi tool turret with live tooling around at high speed, The motor size required for direct drive for this application would be enormous.
Mazak, for example, have made use of timing belt drive/reduction in the past, which is both accurate, simple and fairly cheap.
BTW, I did not mention steppers, I cannot recall any large commercial CNC MTB that has used them in the past.
Al.

[thebodger@roger]

Al:
Hello
As far as industrial grade CNC's with steppers, the old BTC II machines from bridgeport were equiped with steppers and Textron controls. We had them when I worked for the McDonnel Douglas aircraft plant. They were very limited in top speed and torque at higher speeds, which is typical of stepper anyway, but they got the job done usually . The other drawback of belt drive systems is belt flex in a system that requires a certian degree of high accuracy. This is not a problem, usually in most applications, but it can be. In fact MAHO from Germany uses a belt drive with linear scales! and it works well surprisingly, but the belts are very short, keeping flex to an absolute minimum. They use belts simply for compactness, it makes the machine footprint small, as they mount the motor either under or over the said axis, and the pulley ratios are close to 1:1, so torque is not the reason for belts.

[Al_The_Man]

Yes, I know some bridgeports and some others were equipped with steppers, the Bandit controller etc. But they were not really serious players.
I was mainly referring to the larger commercial systems.
Many of the belts systems now like BrecoFlex and GoodYear use steel belted or other technologies to reduce stretch or flex.
See the Goodyear line of Eagle & Falcon transmission belts for example.
I have had good results with timing belt or low backlash planetary gear reducers, every system has to be assessed on what the requirements are as to accuracy.
For example, I recently used a 100:1 planetary reduction on a rack and pinion Gas cutting gantry which which weighed ~400lbs with all 6 torches.
This was gas cutting, but still accurate.
Al.