Crane maintenance and problems 3

[DiamondDave]

Hi guys, a few questions on crane maintenance. I have worked in maintenance for 15 years often called to maintain cranes and other plant and to be honest its really apparent to me that some things are not done properly and my own knowledge is lacking. Some things & practices of others dont sit right with me but Id like to know more.


So

1, what effects can a sudden decrease in load have? For example using a crane to loft something that is seized and the load suddently releasing?

2, what actually stops a crane load running away from itself when lowering under motor power?

3, can a crane with a worm gear box ever drop a load? as I understand in theory a worm should not turn a wheel.

4, what could cause a plummer block on a crane long travel drive shaft to keep coming loose around the wheel area? I suspect it is wear or misalignment on couplings or the wheel at the opposite end not actually being driven thus causing one side to tske the entire drive and the crane to skew?


Any mechanical tils would be appreciated when it comes.to cranes. Common faults and their causes.

A major issue in my workplace is "fixing" things without solving the root cause and thus going back to it a few weeks later.

 

[zeusfaber]

1. Hold tight and watch this. Sudden release of load when the hook broke during a load test causing catastrophic failure of the remaining structure of a heavy-lift crane. Complicated by the fact it was installed on a barge, but a stark reminder that sudden load releases are dangerous.

2. If the motor is hydraulic, then the over-run protection usually comes from a counterbalance valve in the motor return line.

Other people will be able to answer 3 and 4 better than I can.

A.

 

[TugboatEng]

1. It depends on the type of crane. Hydraulic and gantry cranes shouldn't have serious consequences. Tower cranes with moveable counterbalance weights will have serious problems.

2. Crane winches have brakes. If the brake fails the operator would have to balance the load with the hoist motor.

3. Worm gears, contrary to popular belief, do not reliably hold loads and their availability to do so diminishes with wear. Worm gear hoists still have brakes.

4. An image or drawing of this plummer block bearing arrangement could help us spot problems you may not recognize.

 

[DiamondDave]

Thanks for the responses guys, im mainly talking about overhead cranes.

Elecrrical motors in which the brakes lift off completely during operation from what I see they dont control load speed so Is this just controlled by thr motor itself? Is this what stops loads running away?

I have attached a very crude drawing. Basically the black square is the motor the black line represents a drive shaft going to either side of a overhead crane to drive the wheels on either side. The red marks represent couplings and the green marks plummer blocks. At the end you will see 2 plumber blocks this would have a pinion on the shaft in-between them which provides the drive to the wheels which have a large gear ring attached.

It is these 2 plummer blocks which are often working lose, so my suspicion is something in the drive assembly causing them to do so.

 

[desertfox]

Hi DiamondDave

It could just be that the bolts holding the Plummer blocks are not tightened sufficiently and that would be my first port of call
That said is there any information given regarding how tight the bolts should be in the maintenance manual? Also does the manual ask you to periodically check these bolts for working loose? If the latter is a check in the manual then the manufacturer acknowledges the fact these bolts will work loose over a period of time.
Bolts subject to varying loads and or vibration do work loose over time if not correctly pre loaded.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[TugboatEng]

When a hoist is lifting or lowering, the load is held controlled entirely by the motor. It's only when the motor is stopped that the brake sets.

If the bolts for the Plummer blocks keep coming loose consider a few approaches:

Look in to soft starts and stops to minimize mechanical forces on the system.

Increase grip length by adding spacers and using longer bolts.

Drill and install dowel or taper pins

Ream the existing holes and use body bound bolts.

 

[DiamondDave]

Thank you for the responses guys.

So there are 2 cranes of this type on this rail. I have previously been at it and noticed the bolts were lose and tightened them myself. I believe on occasions it has been the bolts holding the top half of the plummer block working loose and on other occasions the bolts securing the block to the crane body have worked lose.

When I checked the crane I found the coupling at the opposite end to not be connecting thus the driving force was going entirely through the end that worked lose. I have been told this has been fixed but i'm not convinced, and if it has I still think there is some kind of fault in the drive shaft assembly now that keeps this side working lose.

It does not happen with the other crane.

 

[rather_be_riding]

Cranes are pretty much all I do day in and out in the Australian market. Things may be somewhat different wherever you may be but I'm making an assumption that things are overwhelmingly similar.

Most overheads (better referred to as bridge cranes IMHO as overhead is a rather poor descriptor) are electrically powered these days and very few have worm gear boxes intended to 'self-brake'. On the way down, the motor and gearing ratio ensures the speed provided. MOST cranes are not fitted with overspeed devices such as you may find on elevators for example. Most brakes are used for deacceleration and load holding purposes only.

Sudden decrease in load with respect to the crane you have specifically in mind may cause little untoward but can often result in pretty poor outcomes. Bridge cranes are typically designed to accommodate a small degree of this behaviour but not a large one. The most common is partial or complete derailment of the bridge. If you have a torsion hoist which is not fitted with proper anti-derailment, you can essentially flip the hoist over the bridge beam. Bad outcome. Similar with top running crabs which can derail as they go into uplift due to the bridge beams bouncing them skywards.

Gearboxes should never be entirely relied upon to prevent load drops. In the case of applications involving suspension over humans etc, we are typically obliged to provide a secondary brake which must be directly connected to the hoist drum or similar. In this way, failure or wear of a gearbox or couplings cannot solely lead to catastrophic failure. Worm boxes are sometimes designed as selfbraking but rarely used as such any longer in cranes. You still really want a brake and they're inefficient means of transmitting power.

Your plummer blocks are interesting. All suggestions above trump what I'm going to offer up in addition. You might also want to check the mesh at each location. The fasteners could be getting 'worked' by poor mesh at points in the mesh procession. This can happen if the mesh is just outright too tight coupled with slightly out-of-round machining but might also occur if heavily greased wheels entrap particles into the mesh which act to try and force the pinion away at the entrapment point. If I had to bet on it, I'd go with poor tensioning though. Old cranes can generate quite a lot of vibration and you're probably looking at fairly short lengths of fasteners which may be struggling to demonstrate much stretch. It used to actually be a recommendation that crane runway beams be fitted with longer bolts with spacer tubes to reduce the chance of loosening.

My final observation is that you seem to be dealing with some reasonably old cranes here. Long travel drives are not generally designed this way any longer and haven't been for about 25 years or more. At some point in time, in most jurisdictions, standards require that the crane is subjected to a major inspection. I suspect you may possibly be due. At that point in time, it's quite common that such things are modernised rather than overhauled; as a result you might often find that you end up with two motors, a smoother drive system (maybe even a VSD). Hoists of that age may be subjected to overhaul or sometimes just outright replaced if they're smaller and cheaper (or if upgrading things like overload protection is too prohibitive). In particular, runways and their alignment can be surprisingly important. Poor alignment can result in unexpectedly high and frequent 'crabbing' loads leading to unpredicted fatigue failures.

My apologies this reply has turned into somewhat of a lecture but I hope there's something within it that is of assistance.

 

[JS_]

Question 1: ..... someone answered

Question 2: ..... someone answered ......Add some more info..electrically operated cranes..Control Brake and a Load Brake one controls lowering of the load and the other brakes when stopped..Holds the load.....Sometimes the control brake is built into the motor using a means of controlling the field winding current example: DC shunt wound regenerative braking....

Question 3: ...... someone answered

Question 4: .........someone answered...............From the sound of it your crane needs removed from service until inspected and brought back into specifications.

I would also check the bolt grade being used for the plumber blocks...

 

[dik]

I'm not aware of it, but can Belleville type washers be used with cranes?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[TugboatEng]

Why?

 

[DiamondDave]

Well guys, the crane ive been most worried about jumped the rails the other day.

I really dont know what to do, it is the same guys doing the "planned maintenance" on these cranes and id say there have been several near misses because of their incompetence and inability to fault f8nd yet nothing seems to be done.

 

[dik]

They're 'lock washers' that work... I was just wondering.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[rather_be_riding]

@dik - yes, sometimes we do. I'm unaware of any limitation against in any code I'm vaguely familiar with.

@DiamondDave - I hope both have been pulled from service and a failure analysis is under way. I imagine along with old cranes, you've got old, badly aligned runways to boot. I wonder if anyone has thought about fatigue and end of life...

 

[Tmoose]

The "preload" that can be exerted by a few bellevilles is pretty low.
The few times I bothered to cipher it out a gawdawful tall stack of conical washers assembled in series was required to approach the clamp force of a well tightened bolt/screw. I meant to go back and compare it to the equivalent increase in grip length, but never did. A couple of standard or HD Bellevilles might as well be (and are) flat washers, until long after the real preload has left the building.

I figure by now all the six-plus faying surfaces in the OP pillow block system are worn and torn up and no longer flat, so the bolts are incapable of creating stable secure joints as originally intended.

 

[FacEngrPE]

1. Everything is a spring. Loss of load can be quite a jolt. Accidentally dropped a 15 ton test weight from a magnet crane (that type of crane is designed to deal with that kind of event). The crane bounced a good bit. The same event on a boom crane at a high angle will usually result in the boom crashing through the boom stops, and bending over backwards. Usually makes the crane a write-off. This is in line with the other's experience.

2, Cranes and hoists all need an intentional way to handle overhauling load. Approaches include breaking resistors (DC hoists, and inverter drives), eddy current brakes (Wound rotor AC), mechanical brakes, hoist drum band brakes, there are others.

3 The few small worm gear hoists I disassembled as an apprentice, all had mechanical load breaks. Worm gears are not by themselves a reliable method of dissipating energy.

4. Fortunately we scrapped most of the cranes having this arrangement. When used on heavy duty cranes, once the bearing faces wear out of flat, re-tightening requirements quickly get to the point where it quickly costs less to replace the travel drive open gears with gearboxes. A skilled millwright can fix the bearing seat out of flat condition, but skilled millwrights are hard to find.

Since you indicate one of the cranes came off of it's rails, it is well past time to check for wheel tread and flange wear, and also rail head wear. If you suspect runway alignment, the requirements can be found here SPECIFICATIONS FOR TOP RUNNING AND UNDER RUNNING SINGLE GIRDER ELECTRIC OVERHEAD CRANES UTILIZING UNDER RUNNING TROLLEY HOIST - NO. 74

This is advertising, but the information on runway alignment is spot on.CMAA & AISC Crane Runway Rail Tolerance

 

[DiamondDave]

Crane has been pulled from service and I understand a specialist company is coming in to inspect the rails.


Ive also pulled a crane in the next bay from service too as ive found excessive wear on the drive wheel rims. I checked the drive shaft couplings(these cranes its one long shaft that drive boy wheels from a central.motor/gearbox) no visible signs of backlash in the couplings.

Also had to adjust the hoist break as the magnet gap was excessive and it was really slamming on hard due to the large gap. I also imagine it wasnt giving full breaking load with the wide gap.

 

[dik]

Thanks, I've not used them for structural purposes... only had them spec'd for electrical equipment. I was just wondering.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik