Stepper Motors
Figure 1: Stepper Motors

The capabilities of the different stack length motors are often confusing to people specifying them and designing them into systems.  The best way to think of the capabilities of the different stack lengths of a particular frame size is to think of the trade-offs.  The smaller MDrive stack length has lower torque that persists to a higher speed.  The larger stack length MDrives have the much higher torque that falls off much more rapidly as speed increases.  This is due to substantially higher inductance in the longer stack lengths.  This higher inductance results from the two things that create the inductance in the first place: iron and copper.  Longer stack length motors have more of both.

Iron and copper are also great at producing magnetism and thereby torque.  By their nature, steppers require the interruption and reversal of motor phase current. The higher the inductance, the more time this takes.   The result is that the current in a phase will be switched before it has been able to rise to the nominal value.  Since full current is never reached, the torque output is lower.

This is all very important when gearing is being selected.  There are times when the application needs more torque than a triple stack can provide, so gearing is added. Since the torque falls off with speed is so severe with the triple, some applications that use gearing are done with single stack motors since they can run faster which is required by the gear ratio.

Steppers are different from other motors in that they produce great torque at low speeds, but at higher speeds, that torque starts to fall off quickly (more so in larger stack sizes).  In other motors, like AC or DC motors, the torque is quite constant from low speeds up to some base speed like 1800 RPM or 3600 RPM.  Above that speed, the torque will then start to fall off.  Since they hold their torque to this higher RPM, they have much higher power because power is the product of both speed and torque.

Installation

To install threaded model nuts, simply hand tighten until the shoulder is flush with the mounting surface. A small amount of Loctite thread compound such as #277 can be used to prevent loosening. Alternatively, a pin can be installed to mechanically lock the threads. Flanged models can be mounted to either the front or rear face of the flange. Before use, it is recommended that the stainless steel preload mechanism is turned so that the caming surfaces move down the ramps. Once play is felt, allow the mechanism to slowly unwind again to establish the proper preload. (It is possible in assembly to inadvertently twist the cam creating excessive drag torque. This procedure will correct this.) Using a lubricant on the lead screw threads is recommended. This extends the life of the nut and reduces heat generation, noise and vibration. TriGEL-300S or TriGEL-1200SC is recommended.

Removal from screw

If it is necessary to remove the nut from your screw, you may lock the mechanism so that it can be immediately reinstalled without re-setting the preload. This can be done by wrapping tape around the junction between the stainless steel cam and the plastic nut halves. This will prevent the cam from turning when the nut is removed from the screw. Remember to remove the tape after installation. For immediate transfer from one screw to another, hold the nut together between your thumb and forefinger so that it cannot expand axially. Remove the nut and install it on the second screw. It may be helpful to prevent the cam from turning with your remaining fingers as you transfer. If the nut becomes disassembled or loses its preload for any reason, follow the steps listed in the assembly procedure below.

Assembly procedure

1 . Insert spring tang into cam slot.

Insert spring tang into cam slot.
Insert spring tang into cam slot.

2 . Ensure that the spring is engaged.

Ensure that the spring is engaged.
Ensure that the spring is engaged.

3. Insert opposite tang into front nut slot or hole (dependent on size). Use the slot or hole that will allow the the cam to be positioned closest to the bottom of the ramp.

Insert opposite tang into front nut slot
Insert opposite tang into front nut slot

4. With washer installed, insert the back nut into the front nut.

Insert the back nut into the front nut

5. With the cam held at the bottom of the ramp, thread the entire nut onto the screw starting with the front nut. After the entire nut is threaded onto the screw, release the cam to observe the gap distance (X on the drawing). The gap distance (X) should be about one-third of the full ramp distance, but no more than half.

Set gap distance

6. If the gap distance is incorrect, unthread the nut just enough to allow the back nut to disengage from the screw. Pull the back nut off and rotate to the next index position and reinsert back into the front nut. With the cam held at the bottom of the ramp, thread the entire nut back onto the screw. Release the cam and verify the correct gap distance. If still not correct repeat this step.

Adjusting gap distance

7. Once the back nut has been properly clocked to yield the correct gap distance, unthread the nut again just enough to disengage the back nut from the screw, but do not remove from the nut. Pull the cam away from the ramp and rotate in the clockwise direction for two ramp settings, then hold the cam at the bottom of the second ramp. Be careful not to allow the back nut to rotate with respect to the front nut while completing this task. With the cam held at the bottom of the second ramp, push the back nut into the front nut and thread the entire nut onto the screw.

Pre-loading the nut

8. The anti-backlash nut is now pre-loaded and fully assembled.

Take the case of a packaging equipment manufacturer with a machine based on a modular design that could be adapted to different boxes depending on the end customer. Let us consider that the primary function of this machine is filling, folding and sealing boxes. Some customers need desiccant bags put in the box before sealing, so a separate modular section will be added to the machine for this purpose.

The desiccant bags come on a reel with a registration hole punched in between each bag. The bags must be separated by cutting at, or near, the registration hole, and then placed one in a box. Typically, air cylinders are used as the feed-to-length and cutter actuators, and a PLC controls the motion. The PLC takes its input from a photocell used to detect the registration hole. Relays and solenoid valves on the output of the PLC control the air which in turn drives the cylinders.

This arrangement would work fine, but to be able to handle different sized desiccant bags the machine needs to be completely reworked. The desiccant bag feeders’ limiting factor is throughput. In addition to this, the air cylinders are the only things that need compressed air on the entire machine. Compressed air processes are known to be noisy, dirty, and they require regular maintenance.

IMS’s AC line driven stepper system would offer a vastly superior stepper solution. A pinch roller driven by an MDrive34AC Plus2 Motion Control stepper motor/driver and control system would replace the feed mechanism. The pneumatic cutter would be replaced with an electric cutter and actuated by a relay controlled by one of eight configurable I/O resident on the MDrive34AC Plus2 . The MDrive34AC Plus2 would be programmed to move at a constant velocity until the registration hole is detected. Once detected, the driver would advance a fixed number of microsteps from that position, then stop and fire the cutter. The desiccant bag cutter module would now work with any size bag without any adjustments or reprogramming required.

This stepper approach would result in a superior system. Precision would be improved and scheduled maintenance reduced to sharpening the cutter blade. Adaptability to different desiccant bags would go from very difficult to completely automatic and transparent. The stepper solution is smaller and requires considerably less wiring and panel space. Throughput would increase due to the vast velocity capability of the AC line driven system. The machine no longer would require compressed air. The stepper system solution would work more efficiently with fewer parts and offers an integrated motion solution in one small package.