Re: [CAD_CAM_EDM_DRO] Opinions on these Reliance 0690 servo motors for BP conversion

The point I was making is that Ebayer's tend to hype their wares. If everything in the ad is accurate, they should work but, I wouldn't mount these on a Bridgeport with 2:1 reduction. With the claim that 1/8 horsepower is ample. How much holding torque does it have? 2.5:1 reduction, that would be roughly 5/16 horsepower to the screw at hold.

If motor technology has improved so drastically that a 32 sized motor is presently much more powerful than older 42's then why do most retrofitters still use 42's?


Ron

Jon Elson <elson@...> wrote:
R Rogers wrote:

>Cal,
>
>The continuous rating is 4 amps and should not be exceeded.
>

Why, then, do they have a 20 A peak rating? The 4 amp rating is the
maximum CONTINUOUS
rating point. You can use the 20 A peak for a couple of seconds per
minute, while
accelerating, without harm.

> The seller claims they are 5.5 lb-in per amp which makes them 22 lb-in motors. Which is hard to believe from a Nema 34 servo with a 3/8 shaft. The ad also goes on to mention that little or no reduction is required. That might be ok for a small mill but definitely not a full sized Bridgeport or clone. If you wound up needing a 4:1 reduction to have enough available torque to the screw, at 1200 rpm on a Gecko. Divide that by 4 and then by 5 tpi, you'd get rapids of like 60 ipm, optimum.
>

Who needs that much torque? Even using the continuous rating, 4 A * 5.5
Lb-In/A = 22 Lb-In. With a 4:1 belt reduction, that is
88 In-Lb at the leadscrew. Assuming a 5 TPI ballscrew, that will
deliver 88/0.0318 = 2767 Lbs of linear force to the table!
Do you really need this much force? How about a 2:1 reduction, for 1383
Lbs force at 120 IPM.

> I'd be careful buying these to use on a Bridgeport. Powering the knee would be out of the question and powering the spindle traverse for Z would require alot of reduction for torque to drill holes. One thing to keep in mind when looking for B'port servos is that no retrofitters that I know of use anything less than Nema 42 motors.
>

Oh? Take a look at a Bridgeport Ez-Trak sometime. They use SEM motors
that are much smaller than NEMA 34 in
diameter. They are actually smaller than these motors in question.
(They are also extremely high-performance
rare-earth magnet motors with stunning specs like 120 A peak!)

> If it were possible to use Nema32's at least a few would be using them because that is the most expensive part of
> the package.
>
>

These motors have a roughly 240 W capacity if limited to 60 V DC. (4 A
* 60 V = 240 W). That is .32 Hp continuous.
I am using 1/8 Hp continuous motors on my Bridgeport, with a 2.5:1
reduction ratio, and it works fine for a home
shop environment. If I go over 60 IPM under manual control, I tend to
have accidents and break tooling. The machine
is already faster than my eyes.

Jon



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