Machine Having 06 Servo vs Machine Having 48 Servo Motors

If Rube Goldberg had designed this machine, it would have only one motor, with God only knows how many PTOs to belts, chains, flapper wheels, etc. Unless you are as clever as Rube, you should probably stick with what you already have, and just RTFM.

I politely disagree.

The number of synchronized AC servomotors doesn't matter, electronic synchronization, like a software-handled gear using encoders and digital servodrives is nothing new. It is beeing used since years in web offest, flexo, etc. print systems. Individual tower drives are just state-of-the art.

As such modular printing presses typically cost several million EUR or USD it's not something you purchase without visiting some companies who use the model you're interested in.

Worldwide there are anyway only very few manufacturers.

BTW I haven't checked the current market but I'm not even sure if centrally driven towers are still manufactured as from an engineering POV it doesn't make much sense where non-mechanical solutions are the better option.

The Mean Time Between Failures (MTBF) is directly related to the number of parts, so yes you are right in that respect but...you need to know the MTBF of EACH part to determine if 6 is better than 48. If the MTBF of each of the 6 servos is more than 8 times greater than each of the 48 servos, then the 6 will theoretically last longer than the 48.

MTBF theories make me smile. :)

Get ready for some surprises when it comes to real-life results.

In the current discussion the MTBF computations don't make much sense as I highly doubt that someone will be able to gather sufficient meaningful data to obtain useable results.

Yup -- real life is always a shock to some, but you but you need to start somewhere!

There could be endless discussions about the usefulness of MTBF computations but I agree that in some cases they give some hints even if quantitative results are often misleading.

In the discussed case the whole system must be taken in account. While with a main longitudinal shaft drive there is only one servomotor there are lots of gears, homocinetic shaft couplings, bearings and other components. In some case you can even use direct servodrives (gearless). Gears requires some sort of maintenance, have shaft seals which leak sooner or later unless replaced preventively...

And precise printing registration (http://en.wikipedia.org/wiki/Printing_registration) is easier to handle electronically than mechanically.

The answer to this question is really simple. In a past life, I spent 6 years working in a roto-gravure plant, so I have some knowledge of the subject.

Both options offer advantages and dis-advantages. Answer this... do you have the in-house technical resources to handle the complexity of the system that does not use a common drive shaft or are you planning on hiring such resources or are they available to you cheaply from local suppliers?

If the answer is no... then buy the common shaft variant, but if you answered yes... the individual drive units offer much more in the way of flexibility on a flexo machine (pun intended). You must (or should) factor in the operation of the press line into your purchasing decisions. Not doing so is negligent.

Indeed I expect those running multi-million web (i.e. non-sheet) printing presses to have competent in-house automaticians. It's not just about the printing presses, there's a lot more and modern machines have advanced controls and drives. In most cases repairs can be done quickly or there's a least one spare tower, or how many times didn't you get your newspaper because of a technical production problem?

If your staff can't use a DSO, monitor a fieldbus or diagnostic a drive with the corresponding software tools you've a problem (of course they need the appropriate tools too).

I've worked on Italian, German and Flemish printing presses. At the most they had 9 motors

What the hell are you printing that needs 48 servo motors

A picture is worth... :)

KBA website (a major printing press manufacturer), 8 main AC servomotors per tower, and here we don't even count small auxiliary drive for inking control or so), for such applications typically servordrives are interconnected for synchronization using some SERCOS communication or some Ethernet flavor:

Image Link:

http://www.kba.com/fileadmin/user_upload/Products/Cortina/27901.jpg

There are Italian brand servo drives and motors being used in both of these machine which we dont have technical support locally. For any kind of problem in these motors and drives which will be beyond our scope, we would have to talk to technical support department of machine manufacturer.

We already have Kollmorgan servo motors working with corrugator machine. Recently we faced a problem in a servo motor of rotary cut-off that when drive tried to run the motor, current increased but motor did not turn even when pulled it out from load. Motor resistance was balanced and insulation test was also ok. at last WE HAD TO SEND IT TO OEM FOR REPAIRING.

What happen when there will be 48servo motors...................................................

Did you check the rotor position feedback signals? With the PC software for the servoamplifier you should be able to check if the resolver or other rotor position feedback signal is correct.

Normally the servomotor and servodrive documentation should be included with the machine documentation set, especially if those are not OEM versions.

If I get a machine without the ORIGINAL documentation of all sreategic off-the-shelf parts, either I request them from the machine manufacturer (as it should be part of the contractual agreement) or get it directly from the part manufacturer. I also recommend to shoot a pic of all accessible rating plates as some of them are now relatively fragile thermo or laserprinted labels rather than thin metallic plates.

Servomotors are not magic, most problems are related to cables and connectors. Also sometimes resolvers and encoders fail (and of course brakes too). Windings fail too from time to time (but there should be sets of PTC or KTY 84 fot stator temperature monitoring).

Servoamplifiers widely vary, some are basic, some include advanced controllers. It's important to have the documentation and, ideally, also the PC software as there are often diagnostic tools included. If there's local display option maybe you should get at least one display (no need to have a display for each drive if they're provided without display) though it depends on conditions, sometimes the PC is sufficient.

Skilled automaticians should be able to troubleshoot servodrive systems even if in most cases, excepted some cabling and connector problems, the servo amplifier, the motor, brake or encoder needs to be replaced. Often there are several identical motors so it's good to keep a spare motor and spare servo amplifier.

AC servo is a known and proven over 25 years old technology (I commissionned the first ones in the late 80's, together with fully digital multiaxis controllers). Most difficult can be the fine tuning if load conditions are tough, that's mainly a question of experience and knowing very well the used drive helps a lot.

See paragraph #2 of answer 6... this statement: "current increased but motor did not turn" points out that you do not have qualified repair people on line. You may need to hire factory trained line tech's, or experienced servo repair people, when you have failures, it will be necessary to have at least 2 people readily available who understand troubleshooting the system.

i think the 48 motored machine(machine 2) is better then the 6 motors machine(machine 1)

with the machine 1 you have to . do the calibration by setting the gear positions on the shafts and more risk of damages (in case of crash in an part of series ) also you will never have control on lash which will accrue with the wear of gears.

but the machine no 2 can be calibrated at the minimum backlash(better precision ) .synchronization is quit easy by setting the encoder pulses .and of course there will be the rapid monitoring of the trouble so you can reach on fault in short time .also may be it is much easier to tailor some functions as your need .

Hi Plimos;

We have two servo motors at top cut-off and two at bottom. When motor 1 of top cut-off got faulted i.e. when drive enabled and tried to runt the motor, it did not run and current increased and drive tripped. At drive output terminal blocks, I swapped the connections of both motors and thus, motor 1 run with drive 2 and vice versa but still motor 1 had same problem which mean that drive and its system was ok problem was in the motor.

Because most of the time top cut-off was run and bottom stand by, so we interchanged the faulted motor of top with ok motor of bot for more tests and found same problem here which made 100% sure that problem was in the motor. Now motor has been sent to machine manufacturer because that was a special motor designed by the OEM on the specs of machine manufacturer which we could not get repaired or buy new from OEM. Waiting for statement from OEM about the problem.

What are the specs of the motor, which manufacturer and which servo amplifier manufacturer?

If it's a mechanically modified motor you've no luck because custom-specific mechanical variations tend to be expensive or even totally unaffordable if you can't order many motors at the same time.

If everything looks standard from a mechanical POV chances are that the motor is not really an OEM version, could also simply be private-labeled. But it's hard to tell without knowing more details. Unless there are mechanical constraints it's often possible to replace an AC servomotor by another one (but wisely chosen, don't just grab the next available with seemingly similar specs) but the commissionning requires some experience, in some cases you can keep the servoamplifier, sometimes you must replace it. More annoying is the case where servoamplifiers have advanced control electronics which can even be like some sort of axis controller.

Overall it will widely depend on how experienced your in-house staff is as servodrives are something specific which requires some practical experience. You don't learn servodrive commissioning by reading books or using Google.

Please find the below images cotaining information regarding servo motor. Drive is Danaher Motion Servo Star 670.

It's a large AC servomotor with integrated resolver but unfortunately the splined shaft end is a customized version. As the shaft is machined during production it's only a factory option, not a modification of an existing off-the-shelf motor or part and custom shafts are typically quite expensive unless you order series and delivery delays are usually very long.
But I'd first check if there's an issue with the resolver and if the resolver is OK I'd check the windings. If protected correctly (i.e. temperature sensors are connected and the servodrives are set up correctly) AC servos are quite robust. It's also very possible that you've simple a resolver wiring problem like a broken conductor or a connector problem. Connector issues are amont the most common failures in the industry.
I'm not sure about Danaher / Kollmorgen, I checked their products some time ago but due to some negative feedback about reliability I choose another manufacturer. Now I can't say if there are really quality problems or if I just happened to talk to people who seemed to have problems, maybe it's also because there are lots of motors around from that manufacturer.