Beyond “capacity”, there is one additional factor to consider, “Duty Cycle”. It would be much like taking your 21 inch Toro and mowing the golf course. Yes it can successfully mow Augusta National, but after a couple of rounds, your new Toro would be ready for the scrap heap. The same goes for a “standard duty” bridge crane when required to make full capacity lifts 24/7! Yes…your 5 ton crane can lift 5 tons, but not in a continuous fashion. And that’s the catch…”continuous”. Continue reading
There are two factors that affect the floor coverage of your Overhead Crane
The Side Approach dictates the usable lateral hook coverage. Note, most of the time this is NOT a symmetrical dimension. There is almost always a short and a long side to the hoist trolley. That is the centerline of the hook to the left trolley bumper and the centerline of the hook to the right bumper. If you required the hook coverage close to the side walls, make sure you talk over this point with the crane/runway supplier.
There is also an equivalent longitudinal dimension from the centerline of the hook to the end of the end truck bumper. This is normally a symmetrical dimension.
In case your end approach and or side approach is especially critical, there is a little know trick of using a “zero clearance” end stop. This can save up to a foot in each direction.
- The usable hook coverage is smaller than the span of the crane, or length of the runway.
- The left to right usable length is normally not symmetrical.
- The crane can be turned 180 degrees to move the short coverage side from under one runway to the other, BUT your power feed will now be on the wrong side and you will have to run a conduit across the bridge to switch the feed end of the crane.
- “Zero clearance” endstops are helpful in both lateral and longitudinal directions.
WHAT DOES RATED CAPACITY REALLY MEAN?
Questions about capacity usually fall into one of two groups;
1. I need to lift 3 tons, should I buy a 5 ton unit for safety reasons?
2. I want to pick up 6 tons with my 5 ton crane. Is that a problem?
Let’s get this over right here and now, you cannot legally exceed the rated capacity of the crane. You may have tried, you may have heard stories, but don’t even try it. Period
Here’s a quick summary on how cranes are engineered.
The CMAA (Crane Manufacturers Association of America) is pretty much the head kahuna of the US crane industry. Every reputable crane builder conforms to these specs, and as a buyer, you should specifically ask if your crane builder conforms to 100% of the CMMA spec 70 (double girder bridge cranes) and/or spec 74 (single girder bridge cranes).
CMAA Spec 70/74 requires crane builders to include a safety factor into their design calculations.
If you add your own “fudge factor”, you are simple making your cranes more expensive with no additional benefit.
Therefore you can reliably pick up the full “Rated Capacity” with any CMAA designed cranes. You need add not additional safety factor, it has already been done for you.
Proper Startup Commissioning
As part of the startup process, your installer is required by OSHA/ASME (American Society of Mechanical Engineers) to perform a load test of the completely installed “crane system”. The crane system includes the hoist, crane, runways and runway support system. OSHA/ASME requires the load test to be performed at 120% of full load capacity.
Conversely, you are limited to lifting 5 tons and no more than 5 tons on a 5 ton rated capacity crane. Most new hoists include an electrical overload detection that provides protection against lifts in excess of 110% of the rated capacity. This additional 10% is not what it seems, the initial startup shock will use up most of the 10%. If the overload protection were set to 101%, it would frequently refuse to lift the legitimate 5 ton lifts.
Bottom line is that you have a full rated capacity of the crane, but no more.
1. CMAA Specs
Make sure the crane your buying fully conforms to either CMAA Spec 70 (for double girder cranes) or CMAA Spec 74 (for single girder cranes). Don’t assume, ask the questions and write it into your purchase order/contract.
2. Rated Capacity
You can safely lift up to the Rated Capacity as marked on the side of the crane. Don’t forget that the below hook lifting equipment is part of the “live load”. In other words, chains, load bars, etc.
3. Duty Cycle
In spite of the fact that you can use the full Rated Capacity of the Crane, you can’t do it 60 times an hour. This is where Duty Cycle comes into consideration and that will be covered in detail in another post.
A copy of the OSHA/ASME Load Test specs is located on the Dearborn Crane website at www.DearbornCrane.com
This issue may be too elementary for many of you, but you would be surprised at how many people incorrectly interchange one or more of the following terms. This results in the customer meaning one thing while the design engineer is thinking of something entirely different.
The following quick review will make sure we’re all on the same page and you get exactly what you’re looking for.
Bridge Crane = Overhead Crane = Overhead Bridge Crane
The main travelling structure which spans the width of the bay is the Overhead Bridge Crane. The bridge consists of two end trucks and a bridge beam (bridging between the two end trucks).
Located on either end of the bridge beam are the end trucks. They provide a connection point between the bridge beam and the house the wheels on which the entire crane travels. These wheels ride on the runway beam allowing access to the entire length of the building.
The horizontal beam of the crane bridge supports the hoist. There can be either one or two bridge beams, but we’ll get into that in a later issue.
The trolley carries the hoist across the width of the building. It traverses along the bridge girder covering the lateral motion. Remember, the lengthwise travel is the bridge motion.
The hoist is mounted to the trolley and performs the actual lifting function.
The runways are the gray item in the picture. They provide the rolling surface for the bridge cranes to travel the length of the building.
- Bridge Crane, Overhead Crane and Overhead Bridge Crane are different titles for the exact same thing.
- Horizontal Motion is the bridge crane travel (lengthwise) and trolley travel (widthwise). Lifting motion (up/down) is the hoist motion.
- Terminology matrix
|Building Width||Building Length||Building Height|
|Motions||Trolley travel||Crane travel||Hoisting motion|
|Hook coverage||Bridge Crane span||Runway length||Lift|
I was just reading a recent edition of Inc. Magazine and I ran across an article about Circuit City and their recent liquidation. A guest columnist suggested that the demise of Circuit City was in fact not the economy, but rather the result of a business that failed to serve it’s customers. He told of how in October of 2007, Circuit City fired 3,400 of it’s most experienced employees and replaced them with near-minimum wage, untrained workers.
He went on to tell about a great and booming retailer, B&H Photo in New York City, and how they exhibit three significant factors:
1. Product Knowledge – An in depth product knowledge of their full product line. A knowledge that transforms them from a sales clerk to a consultant.
2. Price Reducing Suggestions – A willingness to use this vast library of experience to expand or even reduce the size of the sale, depending on the interest of the customer.
3. Competitive Pricing – Because of their size, they have the volume to always provide a competitive quote.
After reading the article, I felt very good that we at Dearborn Overhead Crane strive to meet these 3 qualities. I can promise you that after the initial hello, every person you talk to here has a long history of experience with overhead cranes and hoists. The secret to our experience was discovered by my father and repeatedly illustrated to me in various ways. The secret is to create a company in which people like to work and stay a long time. Nothing compares to simple, old fashioned experience and the only way you can get it is to be on the job for many years.
So the next time you call, be prepared to take a little extra time and make sure to ask those questions you were reluctant to ask others. There’s a good chance we’ll have the answer at hand and if not, we have the largest library of crane related books, articles and specs available.
If you would like to read the Inc. article, here’s the link: Inc. Magazine – It Isn’t the Economy, Stupid
Price only becomes a point of measure when you know exactly what you are buying, and saying both quotes use a 10 ton capacity hoist and have a 50 foot span is not enough!
It is a rare occasion that an inquiry for a new crane system even addresses the crane duty cycle. Let me assure you, this is one of the three most important questions to be answered (if not the single most important).
Let me give you an actual example. I once was asked to provide two 10 ton cranes to operate on a single runway. The first crane was a bridge crane used to feed the charging system in a foundry. The crane ran at full capacity, 24/7 and had a magnet. If the crane went down, several hundred people went home and they had to “drop the furnace” (purge it of all hot metal). It was cab controlled with an optional radio control. The crane cost a little over $300,000 and was rated as a class E crane. Compare this to a maintenance crane, pendant operated, same capacity, same span. The crane was used for maintenance once a year. It cost $35,000, but it was a low end Class C crane. Two cranes with the same capacity, the same span, the same lift height and almost 10x difference in price!!!
So here’s the CMAA classification system:
CMAA Crane Service Classes
CMAA has established crane service classes so that the most economical crane for a particular installation may be specified in accordance with Specifications for Top Running Bridge & Gantry Type Multiple Girder Electric Overhead Traveling Cranes-No. 70 or Specifications for Top Running and Under Running Single Girder Electric Overhead Cranes Utilizing Under Running Trolley Hoist-No. 74. The crane service classification is based on the load spectrum reflecting the actual service conditions as closely as possible. The CMAA
Crane Service Classes are as follows:
CLASS A (STANDBY OR INFREQUENT SERVICE)
This service class covers cranes which may be used in installations such as power houses, public utilities, turbine rooms, motor rooms and transformer stations where precise handling of equipment at slow speeds with long, idle periods between lifts are required. Capacity loads may be handled for initial installation of equipment and for infrequent maintenance.
CLASS B (LIGHT SERVICE)
This service covers cranes which may be used in repair shops, light assembly operations, service buildings, light warehousing, etc. where service requirements are light and the speed is slow. Loads
may vary from no load to occasional full rated loads with two to five lifts per hour, averaging ten feet per lift.
CLASS C (MODERATE SERVICE)
This service covers cranes which may be used in machine shops or paper mill machine rooms, etc. where service requirements are moderate. In this type of service the crane will handle loads which average 50 percent of the rated capacity with 5 to 10 lifts per hour, averaging 15 feet, not over 50 percent of the lift at rated capacity.
CLASS D (HEAVY SERVICE)
This service covers cranes which may be used in heavy machine shops, foundries, fabricating plants, steel warehouses, container yards, lumber mills, etc., and standard duty bucket and magnet operations where heavy duty production is required. In this type of service, loads approaching 50 percent of the rated capacity will be handled constantly during the working period. High speeds are desirable for this type of service with 10 to 20 lifts per hour averaging 15 feet, not over 65 percent of the lifts at rated capacity.
CLASS E (SEVERE SERVICE)
This type of service requires a crane capable of handling loads approaching a rated capacity throughout its life. Applications may include magnet, bucket, magnet/bucket combination cranes for scrap yards, cement mills, lumber mills, fertilizer plants, container handling, etc., with twenty or more lifts per hour at or near the rated capacity.
CLASS F (CONTINUOUS SEVERE SERVICE)
This type of service requires a crane capable of handling loads approaching rated capacity continuously under severe service conditions throughout its life. Applications may include custom designed specialty cranes essential to performing the critical work tasks affecting the total production facility. These cranes must provide the highest reliability with special attention to ease of maintenance features.
This information has been presented for reference purposes only. For more information regarding load spectrum, mean effective load factors, load classes, load cycles and how these relate to the determination of crane service classes, please refer to Specifications for Top Running Bridge & Gantry Type Multiple Girder Electric Overhead Traveling Cranes-No. 70 or Specifications for Top Running and Under Running Single Girder Electric Overhead Cranes Utilizing Under Running Trolley Hoist-No. 74. These documents are available for purchase online at http://www.mhia.org/bookstore or through the Literature Department at 704-676-1190.
After you digest this, you’re not done yet. Next week I will list the dozen specific issues that you MUST include to protect yourself in a buyers spec to “hold your suppliers feet to the fire”. It’s not enough to say, I want it class D. You need to let your suppliers know that you are aware of what a “D” means and you will be checking them.
So here’s the one rule that would be more effective in making a safe work environment than all of the rest of the crane and hoist rules added together.
“Thou shall have overload protection on all lifting devices.”
There it is, just nine words. You may have thought that a mechanical or electrical device to prevent the operator from lifting loads in excess of the safe capacity was already required, but it’s not!!!
Not in OSHA, not in CMAA (Crane Manufacturers Association of America), not in HMI (Hoist Manufacturers Institute)…nowhere. Worse yet, it is such a common sense idea, most crane and hoists buyers just assume that when they insist that their equipment be OSHA and/or CMAA compliant, it will meet this most rudimentary requirement. Well it’s not. Yes, there are rules that prohibit you from lifting loads in excess of the rated capacity of the crane, but no rules that require this to be built into the crane.
I have even met a few buyers that insist that they don’t need overload protection, because they never get loads that exceed the rated capacity of their crane or hoist. Although they may never get larger loads, they do get that one trucker that forgets to take off all the chains or the lathe operator that forgets to un-chuck the shaft. Next thing you know, they are lifting the whole lathe.
Bottom line is it’s up to you. Make sure to write into all your purchasing specs that “Overload” protection is required on all new lifting equipment. If the seller says OSHA doesn’t require it, tell them that you do!
Dr. Frazier Crane
PhD in Craneology (Piled High and Deep)
Last week I talked about the folly of OSHA not requiring “overload protection“. I have received a number of questions and let me therefore take a moment to answer them all at once.
- Wire rope hoists are NOT required to have any type of overload protection
- Electric Chain hoists ARE required to have some form of overload protection
- There are several brands of overload protection that will fit almost all brands of hoist
The topic of the day is a particular type of “overload protection”, that involves cranes with two hoists. We frequently are asked to supply a crane that has two hoists for flipping dies. In this case either hoist may be required to lift the full load, lets say 5 tons, but never will the sum total of the lift be 5 tons. In other words, we have to supply a 5 ton crane with two 5 ton hoists. This may sound like a ticking time bomb, but with modern controls, there is an easy and economical answer.
Most hoist manufacturers and several third party after market control manufacturers now supply a small control system that is installed into the hoist control panel that constantly sums the total weight lifted by the two hoists. With this, either hoist can lift the full 5 tons or can share the 5 tons in any combination. As long as the aggregate total does not exceed the 5 ton load limit. the units operation is totally transparent, but in the event 5 tons is exceeded, the hoist will cease to lift and only operate in the down direction.
Dr. Frazier Crane
PhD in Craneology (Piled High and Deep)
One of the most confusing issues regarding Overhead Bridge Crane rules and regulations is, “Who’s the Boss”? In other words, what laws govern the crane owner and for that matter, the crane builder?
I guess you should start with OSHA 1910.179. It quite frankly lays down the law in surprisingly few areas. That is surprisingly few concrete specifics. What it does is, “incorporate by reference”, a couple dozen other reference specifications. In other words, the CMAA 70 and 74 specifications are incorporated by reference, which means they have the same force of law as OSHA 1910.179. That means for example, the two referenced CMAA specs along with about 2 dozen other specs are therefore within the scope of 1910.179.
As if this referenced body of work is not enough, each of the referenced documents, in turn, incorporate by reference other specs. In the case of CMAA, we’re talking another couple of dozen specs which they reference, including such things as the National Electric Code, AWS welding code, etc.
To add one more layer to this circus of specifications, OSHA also “incorporates by reference” the manufacturers “owners manual” as the ultimate authority on that piece of equipment.
So the next time you get asked a question regarding the “legitimate way” to do something on your crane or hoist, be prepared to invest a lot of time wading through a lot of specs. Many of which cost close to a hundred bucks each. Below is a PDF files that is a map to all the related governing bodies and specification for Overhead Bridge Cranes and Hoists. To date, I have 36 listings, I’m sure you will have more to add and would appreciate your input.
Dr. Frazier Crane
PhD in Craneology (Piled High and Deep)
Dearborn Crane is a crane builder and buys the hoists that go on our cranes from various hoist manufacturing companies. More than a decade ago, one of our major hoists suppliers introduced their new line of “up-rated” hoists. At our office, he sat us down and showed us the newly printed catalogs in which all his hoists were now rated as H4 duty cycle, rather than the previous mix of H2, H3 and H4 hoists.
For those of you not acquainted with the HMI’s (Hoist Manufacturers Institute,) duty cycle rating system, HMI is an industry association of member manufacturers that have set down a series of specifications for the hoist industry. The system rates hoists based on a duty cycle based performance with ratings between H1 (lightest duty) to H5 (heaviest duty).
The H3 hoist is considered to be a “standard” duty hoist and the H4 is the beginning of the “heavy” duty hoists. Prior to this time, most manufacturers had a mixed line of offerings including, H2, H3, H4 and H5 hoists, but at this meeting we were being told that this supplier had only H4 and higher hoists from this day forward.
After showing us the new catalog pages with the H4 designations, the conversation went something like this:
Q. How much is the price increase for the new higher rated hoists?
A. No price increase.
Q. Is this a new line of hoists?
A. No, this is the same product line as before.
Q. Larger horsepower motors, better insulation?
A. No, they have the same motors.
Q. Larger ropes?
A. No, same ropes.
Q. Bigger gearboxes, larger contactors….what’s the difference???
A. Quite frankly, the only difference is that the printer changed the catalog pages from H3 to H4. We did some figuring (he probably didn’t use that exact word, but I think it fits here perfectly) and we determined that all our hoists meet the HMI H4 requirements, so we had the catalogs reprinted with every hoist at a minimum of H4…At this point, I’m sure you want to know who would be so unscrupulous. Well let’s put it this way, today I’m afraid you would have an easier time finding a living dinosaur than a H3 hoists in any manufactures catalog! Who’s at fault? Let me give you my list in David Lettermen fashion.
- The manufacturers who seem to be willing to do anything to make a sale today, regardless of warranty costs later and loss of long term reputation in the market place.
- The manufacturers association, which are simply a reflection of its members. If they are willing to craft a spec that is so loose, that it no longer means anything, then why have it??? Let me put it this way, when even a lowly chain hoist has a H4 rating (and they ALL do), then the designation is meaningless. Worse yet, they are miss leading, in that they give the impression of being something they are not, thereby deliberately deceiving and misrepresenting.
- ME AND YOU, the buying public. We have been so taken with marketing hype that we no longer use our head and do our homework. Try buying a small Coke at McDonalds. You can’t, they only have medium, large and extra large. They think that getting a “Super Meal” with a medium drink sounds better than getting a “Super Meal” with a small drink. Even if both of them only have a 12 oz drink?
What can we do? Do your homework, don’t just flip to the back page and read the bottom line, read the whole document. Price should not be the only factor in your decision. If it is, don’t blame the manufacturer for supplying junk, we have all made ourselves into willing participants.
- Read the whole quote, not just the price
- Make a comparison matrix. If you don’t don’t know the critical issues, ask each of your bidders. Yes, they will each highlight their own strong suits, but their combined list should be as comprehensive as you can get.
- Send the new combined matrix back to each of the bidders and have them fill it out. Stipulate that is there are any blanks, they should not bother to re-submit their quote.
- If you’re not sure of their response, put it into your own words and then use the matrix as part of your contract/purchase order.
Is this air tight, no way. But it’s a lot better than what you probably have now. The old AISE (American Institute of Steel Engineers) had buyer generated specifications as opposed to manufacturers written specs. They had their flaws, but were more effective than the nothing we now have.
Dr. Frazier Crane
PhD in Craneology (Piled High and Deep)