It is often "uncomfortable" to pay too much ( or what you may preceive as too much ) but it is usually far worse to not pay enough. A real DC servo, and a 4 brush more specifically will turn smoothly with as little as the power supplied by a single "D" cell battery with no visual or audio signs of "cogging". Try this on a "treadmill motor" and it will be a big eye opener. That big ol fan unit which is usually mounted on the tread mill motor shaft serves double duty as a flywheel to soften this cogging, also the cogging is lessoned by the reduction belt drive, the belts help damp out the vibration. I like those Mitsubishi units Andy linked to - but Buyer beware - There are servos, and there are sever duty servos - you need a good sealed unit that can handle running in coolant if the servo is mounted in an area subject to chips and coolant. A prime example would be the motor used for a 4th axis sitting on a mill table. That unit will get practically submerged in some cases when run in a common VMC type machine. I have a CNC knee mill, and all servos, even the 2 that are not in areas of exposure are fully sealed - brush caps are O-ringed, O-ring plus silicone sheet gaskets on encoder access plates. All wiring is in sealed conduits. This is how the factory designed it - because the manufacturer does not want to have ANY issues within that 5 year warranty and so it is built to last. My machine is 22 years old, and the most common "Cause of death" is that the proprietary CRT's burn out and they have been out of production for at least 12 years. Even if you decide to gut your control cabinet, using the OEM type motors is well matched to the other existing hardware. Newer amps will provide better response and allow faster profiling. I saw a retrofit that used new AC servos that doubled the rapid rates. It appeard to run great for nearly 2 years - then radical failure - the loads had been too much and the bolts holding the "Y" axis ball screw broke - I think that some bolts broke over time but the last few all failed at once and the bolts holding the ballnut let go and the force was enough to pull the bearing through the end support on the far end of the ballscew. The end support was a total loss. The bolt hole threads in the machine casting were distorted and were drilled out and re-tapped the next size for heli-coil inserts. After the repairs were completed the parameters were reset to 125% of what the machines original rapids were, and with less agressive accelleration.
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