Bridge Crane (sort of) Drive Application 2

[elfman]

We have a new job where we will be providing a hoist type of equipment - that typically travels back and forth on a monorail. However, this particular job we are mounting that monorail, or better stated, that monorail will be part of the Bridge of a bridge crane. Typically, for positioning and driving a bridge crane how is it done? Do you just have one motor on one side of the bridge, or a drive motor on each side of the bridge? Do you have an encoder (for positioning) on one side or both? Is it typically driven by one VFD or by two and how do you sync them? What parts of NEC do I need to pay particular attention to? Are there any worthwhile design guides out there that may help us poor engineers who have never done a bridge crane before know a little more about doing bridge cranes?

I am looking for general information and don't have a whole lot of specific information to provide at this point. I believe the total bridge span will be approximately 50'. Monorail equipment will result in a load of approximately 6,000 lbf.

Any help, suggestions, prayers, etc... will be much appreciated.

"To be or not to be, that is the question" - William Shakespeare via Hamlet

 

[MikeHalloran]

You appear to be talking about an underhung single girder bridge crane with a fairly long span.

In days of yore, the trucks were driven by a single motor and a very long cross-shaft. Nowadays you'd more likely find two three phase motors driven by a single VFD.

That pretty much concludes my knowledge of cranes. Were I in your position, I'd call someone who does this stuff for a living, like Ron Fontes of Material Handling Systems,
720 Southwest 4th Court, Dania Beach, FL 33004
(954) 921-1171

He'll be happy to sell you a complete crane, installed, or a kit (where you provide the beams to save shipping costs), or any components you might need.

Mention my name and he'll charge you extra. ;-)





Mike Halloran
Pembroke Pines, FL, USA

 

[jraef]

In a past life I did work on the bridge crane controls for the Boeing facility where the 747, 767, 777 and now 787 jets are assembled. The bridges were 200ft long and 3 could be locked together to make a 600ft long bridge. Each 200ft span was controlled by two 20HP VFD powered motors, one on each end, each motor driving two mechanically linked "trucks", the drive wheel assemblies at either end. There was no shaft position feedback from those drive motors to the VFDs, but they were in Open Loop Vector Control mode with torque control capability. I did not design the system, but we had to make it work ultimately.

The original design was using a control loop in a PLC that created a Master / Slave follower situation for the motors, wherein the Master was a Speed Follower of the carrier control command signal, and the Slave was a Torque Follower of the Master. Then layered on top of that was the fact that torque was being watched from BOTH drives by the PLC, and to avoid having them skew because of wheel slippage, rail geometry, debris / dust buildup etc, whichever drive was producing the lower amount of torque was made the Master (Speed Follower) and the PLC would change back and forth if conditions changed. It worked OK, but was error prone and with 3 bridges locked together the error would build up over time until the bridges skewed on the rails and jammed. So we had to come up with an anti-skew correction method that involved shooting laser distance measuring sensors crisscross from corner to corner on each 200ft span and feed that differential error in the parallelogram geometry as a feed forward correction loop into the Master-Slave loop. It worked, but it was complicated and expensive.

I've had discussions about this with several crane mfrs since then who said that the basic Master-Slave system used originally is a fairly common way of doing this, but the concept of locking 3 x 200ft spans into one span was not, so it added challenges that the basic concept was not intended to have to handle. So you may be able to implement that just fine on a single 50ft span and have it work.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[lukin1977]

The bridge cranes I know uses a motor and speed reducer on each of two opposite wheels
No speed control or feedback is used. Just DOL start

 

[MikeHalloran]

The truck motors are usually three phase. If they're both driven from the same source, they'll tend to stay in step.
Because they're gearmotors, even a couple of revolutions of drift at the motors doesn't amount to much at the wheels. If you start em 'up with the bridge square, and run into the bumpers occasionally, the bridge should stay square.

Additionally, if the runway rails are crowned (most, but not all are), and the wheels are tapered, the bridge will steer itself to be self-centering and the flanges will hardly ever contact the rails.

The crown on the rails is subtle, typically amounting to .010" or so across the top. It's almost imperceptible to the eye, but a sloppy CAD operator can end up with a lot of mysterious mismatches by snapping to the ends of the top arc instead of the midpoint. (Yes, the tapered wheels don't actually ride on the midpoint line of the crown, but the difference is not worth worrying about in an industry where the default tolerance is the width of a dull Sharpie.)











Mike Halloran
Pembroke Pines, FL, USA