The upper limits

Seven years ago, the ASME B30 standards, one of the most important set of hoist engineering guidelines in the US, were changed. For decades the standards stated that hoists should have upper limit switches to prevent a hoist’s hook block rising too far and hitting the hoist unit itself (commonly known as two-blocking).

In 1998, a clause was added that stated that powered wire rope hoists that use a limit switch based on turns of the drum should have an additional, independently-acting upper limit switch. One example would be a gravity-operated switch that is activated when a weight is hit by the rising hook block.

Despite this change, the market appears little changed since then, and although most, if not all, hoist vendors sell an independent limit, many do not offer it as standard.

Which is not to say that standard hoists without an independent limit switch are unsafe. Not only does safety depend on the design of the hoist, the safety of the unit itself also depends on factors beyond the scope of the manufacturers: how well it fits its purpose, and the way it is used, for example.

Gravity-operated limit switches, like extra load brakes, add extra cost, as well as extra safety. Both illustrate the difference in approach between domestic US hoist builders, and those from outside the US.

There are probably three reasons why the gravity upper limit has not become standard equipment: the price pressure of the marketplace, the complexity of the system of technical guidance and regulation in the USA, and rope guides.

On one side are the domestic manufacturers whose hoists can withstand a thermonuclear detonation: companies like Acco, Detroit and Lift-Tec, Ace World Companies, Morris/P&H and Whiting. They often sell double-reeved hoists and do at least some custom-build work. They include a gravity-based limit switch.

On the other side are the US and foreign companies that import European-made hoists: R&M, Demag, Street, R Stahl. These hoists are mostly single-reeved, less robust – and less expensive. They have drum-based limit switches and wire rope guides. They do not generally include a gravity-based limit switch as standard, but seem to be more likely to include a standard lower limit switch.

US and European standards and laws do appear to agree on two points. First, all hoists need to have upper limit stops. Second, these limit stops should not be used every day as part of a manufacturing process. If customers want so-called operational limit switches, these switches should be installed in addition to the emergency limit switch.

Standard product

Not all US vendors sell a hoist with a block-operated independent limit switch as standard. Of course hoists may be built up with optional extras – but that does not mean they will be.

“With a $20,000 order, if you add $350, you’ll lose the order,” says Neal Wilson, president of Texas-based crane builder Proserv/Anchor, referring to the price of an extra upper limit switch. “So people sell what everyone else sells. It’s the way it is today. It’s just stupid.”

According to Joe Gibbs, Acco sales and marketing manager, upper limit switches are often cut out to offer a lower sales quotation. “I am sure that the reason is price,” he says.

R&Mgeneral manager Jim Vandegrift says the issue is all about marketing. “We all look at how to market our product when there is competition,” he says. “If you want to exceed the regulations, so be it, as long as you meet the minimum criteria. If you exceed them, you put that as a feature of the product and sell it. On the other hand, if regulations do not demand it, you have another option – to take it out.” Vandegrift says that the R&M product, which uses geared limit switches, meets the letter and intent of the law.

According to Ace World Companies founder Ace Ghanemi, the competition, especially among single-reeved packaged wire rope hoists, is extreme. The company has just launched its second modularised built-up hoist, which is double-reeved. He explained that the competition was partly why Ace decided to go double-reeved: “There is no way a domestic company can compete mass-producing hoists that competitors can manufacture worldwide. You have got to have a lot of market to mass-produce hoists. The US is just one small market of theirs.”

Others disagree about the importance of price. Alabama-based Deshazo Crane, which buys kit hoists and makes its own, fits a gravity-operated limit switch and a gear-driven limit switch as standard, according to electrical engineer Tom Young. He says that the extra cost is only a slight disadvantage. “It’s not a deal-breaker,” he says.

Hoists are generally not sold in Europe with an independent limit switch, according to Steve Woolley, US sales and components sales manager for the UK’s Street Crane. He says this matters because what is standard in Europe will be sold that way everywhere to keep costs down. “If at all possible, they want to sell standard equipment into the US as well as other markets,” Woolley says.

Until European crane standards are eventually standardised in EN 13000, hoist makers are working to national and regional standards, which are all similar, according to Woolley.

None of the standards appear to call for a weight-operated limit switch. Also, European standards require limit switches to be electromechanical – electronic switches are not allowed for safety reasons, according to Oliver Ganz, SWF wire rope and rope hoist technical support.

The law

The exact legal status of the upper limit appears to be caught somewhere between non-binding technical recommendations of the trade associations and the enforcement organisation, OSHA.

There are three main bodies writing technical guidance in the US overhead crane business: the engineering standards organisations ASME/ANSI (most recent: B30), the overhead crane trade group CMAA, which published specifications on multiple-girder and single-girder cranes last year (Specs 70 and 74), and the steel trade group the Association of Iron and Steel Technology (Specification for EOT Cranes for Steel Mill Service, TR-06, 2000). All three standards are voluntary.

Hoist OEMs have differing attitudes to the voluntary nature of these standards. “We as a manufacturer look at what the industry has accepted, which is B30, and try to meet that specification,” says Acco’s Joe Gibbs. “To me, if you sell a hoist, it should meet that spec. If not, if you deviate from that, then you are doing it on your own and you should say why you’re doing it.”

The ASME/ANSI B30.16 regs were updated to include the requirement for a gravity-operated limit switch in 1998.

But the letter of the law comes from the versions of these standards that are incorporated into the US regulatory agency OSHA, the Occupational Health and Safety Administration. The OSHA crane regs are found in 29 CFR 1910.179, Overhead and Gantry Cranes. The current Federal OSHA standard refers to a version of B30.2 from 1967. Jim Vandegrift at R&M says that the 1967 version of B30.2 makes no mention of the type of limit switch. According to an internal R&M document, the 1967 standard only says: “The hoisting motion of all EOT cranes shall be provided with an overtravel limit switch in the hoisting direction.”

Vandegrift says:“My guess is that if OSHA changed the standard, those manufacturers that don’t offer this as an option or don’t offer it at all would end up providing it on their hoist as standard equipment.”

The very actions of some of the manufacturers are proof enough for some crane builders in the market. “You have to understand that if Stahl doesn’t do it, and Street and R&M don’t do it, it must be okay not to,” says Neal Wilson of Proserv/Anchor.

Federal OSHA did not respond to multiple requests for information by the time of going to press.

Latest standards

The regulations of other standards-making bodies suggest that no particular limit switch is required. Michigan OSHA, for example, may create its own health and safety rules, provided they are as good as Federal OSHA regulations, according to John Brennan, director of the general industry safety and health division of the office of general industry compliance. MIOSHA regs make no reference to the type of limit switch: “All cranes shall have an operable over-travel limit switch in the hoisting direction. The switch shall be located so that it is tripped under all conditions to prevent contact of the hook or block with the hoist.” (Part 18, R 408.11841, (7)).

All the same, OSHA may still hold crane builders to the latest standards. OSHA can use the most recent version of a referenced standard for enforcement, even if the statute does not explicitly refer to it, according to ASME B30 committee secretary Joe Wendler. He says that particularly in the event of an accident, an enthusiastic investigator might turn to the provisions of Section 5 of the OSH Act, or the General Duty Clause, which requires companies to offer safe places of work. “An attorney might ask, ‘How come you didn’t comply with the general standard?’”

Rope guides

The theory behind the ASME rule appears to rest on the absence of rope guides. Rope guides travel along the rope drum and spool the rope on to the correct groove. They are uncommon in the USA, but commonly used in Europe. UK manufacturer Street, for example, does not sell a wire rope hoist without rope guide.

Gear-based limit switches take the position of the rope from the number of turns of the drum. Proximity switches use a magnetic field to determine whether or not a particular groove is filled with rope.

Both of these switches need a secondary check, the argument goes, because of the risk of mis-wrapping. If the wire rope wraps on top of an existing layer of rope instead of in the empty drum grooves, the hoist will think the hook is lower than it actually is, and risk two-blocking. This is because the drum is effectively wider if the second layer of rope winds on top of the first layer of rope than if it is wound around the bare drum.

“That’s valid if you don’t have a rope guide,” Fabio Fiorino, president of R&M, says. “It probably happens rarely, but in a remote situation, it is valid.” He added: “You won’t get overwrapping with a rope guide. That takes care of that issue.”

Other manufacturers disagree.

“Some rope guides are susceptible to breakage under certain operations, some are more of a nuisance when hoisting with a swinging load caused by bridge or trolley movement,” says Jeff Breitrick, vice president and International general manager of Morris Material Handling/P&H.

“A minor modified pull can break the guides. You shouldn’t side pull, but if you do, it creates a problem for the guides,” Breitrick added.

And others feel even more strongly.

“We feel rope guides are a maintenance gimmick to make money for hoist manufacturers, primarily European manufacturers,” says Chuck Melder, general manager of Detroit Hoist. “Because we design drums and fleet angles properly, in a normal straight lift, that everyone should do, the rope should spool properly. Then the rope guide tells when the customer is side pulling,” he says. Still, Detroit installs rope guides on 60% of its cranes.

“I don’t accept that if you had a rope guide you wouldn’t need a gravity-operated limit switch,” Melder says. “Rope guides break off all the time. Our upper limit functions in an emergency because the block has to be present.”

Demag USA mechanical design engineer Craig Black recommends that customers should replace their rope guide when they change the wire rope. Particularly in cases of sidepulling, a rope guide helps preserve the rope by preventing it from rubbing against the drum grooves, he says.

Ian Wing, Street crane technical director, says that rope guide-based limit switches were, in his experience, just as reliable as geared limit switches.

Fiorino at R&M warns that upper limit switches should not be seen as the last word in hoist safety.

“On the other side, people don’t focus on other safety issues, such as having a standard overload detector, or inverters on travel motions to reduce load swing,” Fiorino says.

And he objects to the specific wording of the ASME rule. “You should look at the overall intended performance of the unit, and what it accomplishes, rather than details of how to accomplish it,” he says.

It is perhaps fitting that this partner of European hoist giant Konecranes would state the exact approach that is taken across Europe. In addition to national standards, all manufacturers of equipment sold in Europe are bound to make machines safe by a Machinery Directive. “The Machinery Directive is not specific at all. It is far too general. The onus is on you to put limit switches in place to protect the system. You do your risk assessment to decide what limit switches are appropriate,” says John Howells, engineering manager for UK hoist and crane manufacturer Morris Material Handling Ltd.