my new cheap servos

[Dan S]

I found these servos on the net, and they are dirt cheap, so I just had to get them.

the website where i got them

http://www.cnczone.com/gallery/data/...85myservos.jpg

I posted a message on the gecko forum asking if they where any good and Marriss replied with the following.

“
Use a 36VDC power supply.
No-load RPM at 36VDC = 3,130
(Ke = 11.5, KRPM = 36V / Ke)
80% of no-load RPM = 2,500 RPM
Leadscrew RPM @ 120 IPM, 5TPI = 600RPM
Reduction Ratio = 2,500 RPM / 600 RPM = 4.1666:1, Use 4:1
Kt = 1.351 Ke = 15.5 in-oz / amp
Maximum continuous current = 3 Amps
Maximum continuous torque = 3A * Kt = 46.5 in-oz
Torque on leadscrew = 46.5 in-oz * 4:1 reduction = 186 in-oz
Leadscrew thrust = pi/8 * TPI * in-oz = 365 lbs
Maximum speed at full load = 2500RPM /(ratio * TPI) = 125 IPM
Maximum speed, no-load = 3,130RPM / (ratio * TPI) = 156 IPM
Power delivered to load = RPM * in-oz / 1351 = 86 Watts

Mariss
“

If these servos are even half that good I will be happy.

-Dan

Similar Threads:

• Just In Bobcad-Cam V25 Mill/Lathe/Plasma + tons of EXTRAS. CHEAP CHEAP CHEAP!
• Problem- Propriatary AC servos into universal AC servos with an encoder swap?
• Cheap HF router bits, just plain cheap?
• Why won't cheap servos work?

[ezland00]

I don't think they have encoders. Do they?

[HomeCNC]

I just got my one servo that I ordered to check it out. It looks like they can be modified to hold an encoder. I'm going to reverse the end caps because I like the cap where the wires connect as the front. I will turn off the knurl as well.

[Dan S]

If your using belt drive isn't it better to hook the encoder to the screw, and not the servo?

Jeff

whats knurl??

-Dan

[barbwirebi]

Jeff & Dan

I'm still in the conception phase of my router design and don't really know what I exactly need yet. But I could not pass on this deal!
I bought five.
Guees this resolves my Stepper versus Servo Issue

So how much can I expect to pay for encoders?(.001 resolution)

I want my router to handle 4' x 8' sheets of 3/4" MDF
If I slave two motors together do you guys think I will have enough torque to move a (aluminum)gantry along @ 60 IPM
cutting .500 depth of cut in MDF?

If thesse motors cant do it, perhaps I should build a small mill first.


Thanks in advance
This is a great forum!!
Barbwire Bill

[arvidb]

Originally posted by Dan S
If your using belt drive isn't it better to hook the encoder to the screw, and not the servo?

-Dan

You might get problems with chasing if you hook the encoder to the screw, unless you have a backlash-free coupling between motor and screw.
I.e. the servo driver sees a position error and commands the motor to compensate, the motor compensates and then the position error changes sign, and the motor has to compensate in the opposite direction. This is normal with servos but if you have backlash between motor and encoder the motor will beat back and forth; you will get nasty vibrations and it might overheat the motor.

Arvid

[HomeCNC]

I use USdigital encoders on my servo motors. I get the E2 with a 250 CPR wheel. They work great with the G320 drive from Gecko. The cost of an E2 is $36.00 each. That would make these servo motors cost under $50.00 each. Not counting the machining time needed.

[barbwirebi]

(Post #5 )

Jeff

I want my router to handle 4' x 8' sheets of 3/4" MDF
If I slave two of thesse motors together do you think I will have enough torque to move a (aluminum)gantry along @ 60 IPM
cutting .500 depth of cut in MDF?

Should I build a small mill first?


Thanks
Barbwire Bill

[HomeCNC]

I have an aluminum gantry router using one of my 430 oz/in servo motors belt driven at 2:1 ratio. I am using .2 pitch ball screws and can move my gantry at 200 IPM rapids. Cutting at 60 IPM .5 deep should be fine if you keep the spindle RPM high and you are not trying to do this with a large router bit like 3/4"

I need to do some testing of these 'Cheap servo' motors against my new ones from Poly-sci. My new motors are a bit longer and are 1/2 pound heavier.

[barbwirebi]

Thanks Jeff

I was planning on using the example that "Mariss" gave
.2 pitch with 4:1 reduction
I would be happy with 125 IPM rapid
I also would like to use the "Timing Belt " concept for x,axis drive
But I dont know how much thrust it would use to run with no load.

Thanks
Barbwire Bill

[Snipes44]

Hey Jeff, just curious if you ever got a round to doing some test on the 'cheap servos' ? Was thinking about picking up a couple. Hard to pass up at that price.

-Wes

[mjse]

Hi Dan
What is web address of "gecko forum " .
Thanks

[BeerFizz]

Hi Dan,

Have you gotton your servos working yet and how do they perform?

[HomeCNC]

Hey Jeff, just curious if you ever got a round to doing some test on the 'cheap servos' ? Was thinking about picking up a couple. Hard to pass up at that price.

Yes, I finished my testing and I have the full specs on these motors.

They are the next size smaller than the ones I get new from Poly-sci. They are 360 oz/in peak motors at a peak of 20 amps. This means they have an 18 oz/in per amp rating.

Testing results:

Made a bar that had a hole 1" from the center and placed a wire hook there to attach to a bucket. The only thing I had to add for weight was 45 cal bullets. I had my Gecko drives current limited to around 10 amps. I kept adding weight to the bucket and lifting the bucket with the servo motor. I got to a point where the motor would just lift the weight past horizontal and then the drive would fault and the bar would fall down vertical. I weighed the bucket and it was close to 11 pounds. So it lifted 11 in/lbs with my current limited to around 10 amps.

This is very close to what my Poly-Sci specs say. If you do the math, the spec says 18 oz/in per amp. So 18 * 10 amp is 180 0z/in. Convert that to pounds would be 180 / 16 = 11.25 lbs. I'm a 1/4 pound off. Maybe I don't have the current limit screw set to exactly 10 amps

The perfect power supply for these would be a 36VDC supply @ 20 amps. You could leave the Gecko drives current limit screw all the way up to 20 amps. This way if you needed the current for one axis that is working hard it can take what ever is available from the power supply.

Need less to say, I got a few cases of these motors and encoders. I'm going to offer them on ebay for a very good price. There is some machining work needed to fit the encoders so I will need to figure that into the price as well.

[ullbergm]

Originally posted by HomeCNC
The perfect power supply for these would be a 36VDC supply @ 20 amps. You could leave the Gecko drives current limit screw all the way up to 20 amps. This way if you needed the current for one axis that is working hard it can take what ever is available from the power supply.

Quick question. Why 36VDC?
My knowledge of this stuff is not great and i was wondering why you stop at 36VDC instead of going higher.

Thanks,
Magnus

[HomeCNC]

Quick question. Why 36VDC?

Because the spec sheet from Liton Poly-Sci says the motors terminal voltage can take higher but for constant full time use it should be 36VDC.

[ullbergm]

Originally posted by HomeCNC
Because the spec sheet from Liton Poly-Sci says the motors terminal voltage can take higher but for constant full time use it should be 36VDC.

Well.. i guess that makes sense.. i was looking at the motors earlier in the thread and i didnt see any voltage limits.

thanks.

[Snipes44]

Thanks alot for the info Jeff. I'm going to pick up a set of these before there gone. :-)

[Dan S]

Mjse

You can find the gecko forum at
http://groups.yahoo.com/group/geckodrive/


Beerfizz

I haven’t had a chance to work on my machine yet. I’m a college student living in an apartment at the moment. I don’t even have a shop to work in. I graduate her in a week, so as soon as I can find a job, I’m getting some tools, and I’m going to start building my machine.

[barbwirebi]

Jeff (and all other gurus)

I have read the results of your testing of these motors.
You have found that they are 360 in/oz peak @ 20 Amps peak (18 oz/in per Amp)
The manufacturer calls out a spec "Maximum continuous current = 3 Amps"

First question is , what is the difference between the spec "peak oz/in" and the spec "Maximum continuous current "?

I am planning to use a timing belt drive with a 9:1 step down ratio
At peak amps my Thrust calculates out to 359 ft/lbs
Where: Thrust = PI/8 * TPI * in/oz
TPI(Threads per inch) = .2823

At "Maximum continuous current = 3 Amps" my Thrust = 53.8 ft/lbs
This I think is the maximum thrust I can continuously push a cutter in a single axis.

I think I'm ok , because I will only be using Bishop Wiescarver V-Groove roller bearings and slides. In other words I will not have a super beefy set up anyway, so I am planning on using light depths of cut with a lot of cutter rpm.

If I understand correctly , peak thrust is needed to get everything moving at a fast acceleration rate. And the peak Thrust is also needed again at deceleration for quick direction changes. Am I "out to lunch" with this thinking?

Jeff you also say in your test " The perfect power supply for these motors would be a 36VDC supply @ 20 Amps
I plan on having a 4th rotary axis
If I understand correctly, I don't need my supply to provide 20amps per axis (total 80 amps).I have heard I will probably never max all axis out at once .
So are you saying a supply of 20 amps total will be adequate?
Would 40 amps total be better or is that overkill?

Thanks to all
Bill