NOTE: If you are new to this overhead crane thing and haven’t read our GUIDE TO OVERHEAD CRANE TERMINOLOGY, then please do so first as it describes many of the technical terms that are used here.
As a manufacturer of high quality overhead cranes one of the most common questions we come across when trying to sell our product goes something like this : “your quote is higher than company X, and they are saying that their product is just as high of quality as yours, so why would we go with yours?’’ The fact of the matter is, everyone claims to have a high quality product, even though, in my humble opinion, very few of them do. I have yet to see advertising slogans such as “not too bad, but cheap” or “OK for the money” in any of our competitors brochures, even though that accurately explains ¾ of the market. So how is someone who is not an industry insider to decide what brand of crane to buy (and who to buy it from)? Read on…
Easily the most overlooked specification, duty cycle, or duty class, likely has more bearing on reliability than any other specification. The easiest way to make your crane break down a lot is to use it more than it was designed to be used. Duty cycle is especially important since it is next to impossible to upgrade it after the fact. Every component on an overhead crane is sized according to both capacity and duty cycle. From hoist motor, brake, wire rope size, and wheel size, all have a bearing on reliability AND duty cycle, just look at the drawings below to see how two 5,000Kg hoists can be dramatically different due to duty class:
Now granted, this is not a perfect comparison due to the fact that one is an under running and the other is a top running style trolley, but outside of that, just look at the hoist portion and you will see that:
- The hoist motor is considerably larger on the 5m hoist
- The wire rope is 50% larger and it is a 4 fall rather than a 2 fall hoist, so it would take approximately 800% more force to break.
- The frame is not only larger but is made from thicker gauge material on the 5m hoist.
So it should be no surprise to anyone that the 5m hoist in the above example, if used in exactly the same manner as the 2m hoist, would likely be much more reliable. But this is not how a hoist manufacturer expects you to use a hoist. They expect the 5m hoist to be used 800% more than the 2m hoist, if they are consistently lifting the same weight. So, the #1 way to increase the reliability of your potential crane purchase is to increase the duty cycle of the hoist you buy past what is required; conversely the #1 way to reduce the reliability of your crane is to buy one that is a lower duty cycle than what you need. Just look at the diagram below to see how much use affects the allowable capacity of a hoist:
This hoist is de-rated from 20,000Kg down to only 6,300Kg when being used 8x as much (2m to 5m duty cycle), this is how dramatically use affects duty cycle. How we recommend to go about this is 1) Determine your required hoist duty class (See our Primer on Overhead Cranes) 2) Increase it by at least 1 duty class, more if you are unsure of your estimates or if you plan on expanding its use sometime in the future.
Another often time overlooked spec that can be useful in determining a cranes reliability is the amount of wear that a manufacturer allows for components. The easiest of these to determine is the wire rope, which is likely to be one of the most commonly replaced wear parts on your overhead crane. On the chart below we can see that even though the first three examples are all 2m rated hoists, the amount of wear allowed varies considerably. When you take into account the fact that in example 1 the wire rope is just barely strong enough to pass the minimum allowable breaking strength for an overhead crane the 3% allowable wear makes sense. This is an example of a manufacturer that engineers things right to the edge, making it’s components just strong enough and then leaving it to the customer to have to change these components much more often than other manufacturers (it seems to be working for them, they are the largest crane manufacturer in the world… hint hint.) It is likely that a manufacturer would extend this low threshold to allowable wear throughout other components as well, the wire rope that a manufacturer chooses tends to be a good indicator in this regard.
|HOIST CAPACITY||WIRE ROPE SIZE||WIRE ROPE CONSTRUCTION||BREAKING STRENGTH||DUTY CLASS
|10,000KG||11MM||8 x 19||84.4kN||2m/D||3%|
|10,000KG||11MM||8 x 25||110.9kN||2m/D||5%|
|10,000KG||12MM||6 x 36||109.7kN||2m/D||10%|
|10,000KG||12MM||8 x 25||145.2kN||4m/E||10%|
INVERTERS: THE BEST, AND WORST, WAY TO IMPROVE THE RELIABILITY OF YOUR OVERHEAD CRANE.
The modern inverter is easily the most significant development in motor control technology in the last 20 years. These little black boxes can solve many of the problems that have been plaguing induction motors for over a century, and if not done properly can cause even more of them. Far too often inverters are used for all the wrong reasons, such as:
- To get away with using a smaller drive motor than would have been acceptable otherwise, which can lead to durability issues.
- To reduce the need for a quality braking system, braking systems can often be reduced in size, but should still be of high quality).
- To increase the speed of a motor past what is possible with line voltage, reducing its reliability.
- To add proprietary functions to an overhead crane that often mean that you can only go back to the dealer for service.
- Inverters do add complexity, which can be a negative when maintenance personnel is insufficiently trained.
On the other side of the coin, inverters can be used to solve these common issues:
- Speeds can be adjusted infinitely between the min and max allowable.
- Brake wear is nonexistent on properly designed inverter controlled systems..
- Motor induced gear train stress is greatly reduced, and lifespan greatly increased.
- Motor condition and temperature can be monitored much more accurately, reducing the likeliness of overheating.
- Speeds can be increased during light loads, increasing productivity.
- Features such as sway control or sway reduction can increase control of the load.
- Loads can be monitored without the need of a load cell, which requires periodic calibration.
- Motor voltage fluctuations can be all but eliminated (this is one of the leading causes of motor failure).
- Inverters can store ‘Error Codes’ which can help diagnose intermittent problems that don’t always show themselves when a technician is present.
- Virtual limit switches can be programmed into hoist inverters, with no chance of them ever wearing out.
As you can see there are a whole lot more points on the plus side of inverters than the negative side, the problem is again, manufacturer’s that size things ‘just big enough,’ this will often lead to premature inverter failure along with ‘nuisance tripping.’ An easy way to tell if a manufacturer is undersizing their drives is again to look at the spec sheet, if there are restrictions on acceleration and deceleration times it means the drives are undersized. A one second accel/decel time is reasonable, but if you can’t set the acceleration to quicker than three seconds then it means the drive can’t handle the extra current required to accelerate a load quickly.
WHO TO BUY FROM
If you bring up some of the above points to you local XYZ Crane Co. sales person and they look at you like you have a third eye, I would say you should keep on looking. I would at very least ask them about other manufacturers products, and fact check them as much as possible just to see their level of knowledge (and level of BS). Getting a good product is as much about getting the specifications right as anything and the temptation is always there to sell you the same product they have been recommending to everyone else. But if you are willing to do a little homework, and spend a little extra time and possibly money it is sure to save you a lot of grief later on. I wouldn’t waste my time on a crane company that wasn’t willing to spend a little time getting to know what I need.
NEIL M. DORIS
CANADIAN CUSTOM ELECTRICAL LTD.