hi my friends

we are making a plasma cnc machine with following spicifications :
1- Ac servo motor
2- rack and pinion system
3- with pulse command to ac servo motor

but we have a problems :
the servo motors work properly when they are without load ( not coupling to rack ) but when we coupling it with rack with direct coupling the servo motor starts sudden vibration.
we use a Moule 2 rack and 22 teeth pinion with direct coupling

our motor is 750 w and 1.5 kw

my coworker say that problem is from direct coupling if we must use a gear box.
does it is correct oponion?

"Cogging" is a problem on some motors due to the normal accelerations and decelerations that occur as the magnetic fields are created and collapsed.

Some grinders are quite susceptable to any motor cogging - they put chatter into the part when you're trying to grind it very smooth. The problem can be MORE pronounced with direct coupling as there is nothing between the motor and the part to absorb the speed fluctuations.

Asside from going to at DC motor (these tend to run smoother, especially if the armature is skewed along with the pole pieces in the field), the AC servo should run smoothly if it is being PROPERLY driven with the appropriate AC current.

Since the DC current is continuous and not fluctuating as in AC motors, armature rotation tends to be smoother and, depending on pole count and motor integrity, cog free.

Adding a flywheel is a way to help reduce the cogging and also to help smoothen out the armature pulsing in almost any motor. Running the motor at a higher speed with more gear reduction also helps smooth things out, especialy when you're running the motor at partial speed. Part speed, especially when VFD's are being used is when some cogging is MUCh more pronounced.

Be careful too as some VFD drives are not good at converting single phase to 3 phase in a fashion that the AC servo motor "likes".

The use of a "slinky" type of spring coupling between the motor and whatever is being driven can result in some real herky-jerky motion - this definitely NOT a good way to contnect the motor to whatever is being driven.

I"d look a bit more into your system before you start looking at couplings and other "fixes". You could be overlooking something much more important.