Updating crane systems for optimal performance

Key concepts Crane systems are usually updated to increase capacity or match changing production requirements. The system consists of a hoist, trolley, and crane bridge.

12/01/2000


&HEADLINE>Key concepts&/HEADLINE>

Crane systems are usually updated to increase capacity or match changing production requirements.

The system consists of a hoist, trolley, and crane bridge.

Type of bridge, operating envelope, environmental conditions, and performance parameters must be determined.

An overhead crane bridge can lead a long, productive life. For example, an electric crane built for the Cheyenne Railroad in 1884 is still in operation. Longevity of the overhead crane bridge is extremely important because it is often the most expensive part of the system.

This article offers several tips on how to update the crane bridge system by replacing components, such as the hoist, controls, and electrification.

When to update

The two most common reasons for updating an overhead crane system are to increase capacity and accommodate changing requirements of production or other processes.

Process machinery is becoming increasingly faster, while many end products are getting heavier. Perhaps the 3-ton steel coils produced in the past now weigh up to 5 tons because of production changes. Rather than replacing the whole crane system to handle the load increase, it may be possible to install a higher capacity hoist and upgrade the bridge.

Faster throughput and other changes in production flow also affect system requirements, such as speed and duty cycle. The hoist may now need to lift the prescribed load once every 5 min, rather than once every 60 min. Weight of the load, number of times the hoist works per hour, and number of hours or shifts per day all contribute to determining equipment needs.

Upgrading to a hoist with a higher service capacity and faster hoisting speed that can handle more cycles per hour effectively improves the whole overhead lifting system.

Consider a technology upgrade

The need to replace a hoist provides an excellent opportunity to think about additional ways to modernize the crane system. Recent developments in control technology and improvements in electrification are worth examining.

While older magnetic contactor controls abruptly switched power to the hoist on and off buffered by resistors, newer products employ variable frequency drives that ease power to the hoist motor through programmed acceleration and deceleration. There is no surge of electricity so the motor runs cooler, lasts longer, and experiences fewer problems. This kind of control also avoids shock loading to the drive train and gives better load control, because hoist speed is ramped up slowly and smoothly and loads are positioned more precisely with less swing.

Typically, older hoists are controlled by a pushbutton station that hangs from the hoist or bridge. Some crane systems employ cab controls, with the operator sitting in an overhead compartment that rides along with the crane. Obviously, these types of controls limit the operator's mobility. Radio systems where the operator has the transmitter in hand and can control all crane motions from almost anywhere in the bay may be a better choice.

It might also be wise to modernize with a festoon cable system. Some old cranes use bare copper wires or steel angles to carry power to the hoist and trolley. (OSHA now prohibits exposed high-voltage systems like these on new equipment.) Festoon cables provide a safer, more consistent power supply.

Specifying a new hoist

A manufacturer needs detailed information in order to supply the right hoist. Whether custom or straight from the catalog, the hoist must perform satisfactorily and accommodate the layout, confines, and obstructions of the work area.

Here is a checklist of questions the specifications should answer, along with some examples of how the answers might influence the type of hoist chosen. Giving each manufacturer the same data helps evaluate competitive bids.

  • What type of crane bridge will the hoist run on-single or double girder? This decision determines whether the hoist should be a monorail type running on the bottom flange of a single girder bridge beam, or a top-running model moving on top of a double-girder crane.

      • If it is a single-girder crane, what type of beam will the hoist be suspended from? What are the depth, flange width, and flange thickness of the beam? This information is required to assure that the hoist's trolley fits on the beam.

          • If it is a double-girder crane, note the size, type, and center-to-center spacing of the rails the trolley will run on. In addition, what is the width of the bridge girders, and what is the distance between their inside edges? These dimensions are needed to assure that the wire ropes of the hoist won't rub against the edges of the girders at the high or low points of the lift, and that the trolley will fit on the rails and clear the bridge girders.

              • What is the envelope that the hoist will pass through? Specify the size and location of any obstructions in the hoist's path, such as lights, pipes, ventilation systems, sprinkler heads, roof trusses, etc., to make sure the new hoist clears them.

                  • What type of environment will the hoist operate in?

                    • - Is the hoist indoors or outdoors? Equipment used outdoors needs to be weatherproof so that water and moisture won't damage the electrical components.

                      - Is it outside in snow country? If so, weatherproofing, plus shielded cross conductors to prevent ice and snow buildup that can cause sagging or breakage, are required.

                      - Is the environment dusty? Control panels need to be dusttight.

                      - Is it a high-heat environment? If the hoist will be used in a place where the ambient temperature is high-such as a foundry, paper mill, or hot climate-this may determine the type of motor or controls to use. If the hoist is exposed to radiant heat, shields for the motors, controls, and drive trains are required.

                      - Is the moisture content high? Condensing moisture in extremely humid climates can damage motors. Use moistureproof controls and motors, possibly with heaters.

                      - Is the atmosphere corrosive? Hoists used where etching, cleaning, plating, or pickling processes are performed; or where they are exposed to acid baths or caustic fumes, need special paint and motors, as well as control enclosures, to prevent corrosion.

                      - Is the environment explosive or hazardous? Explosionproof controls, motors, and brakes are essential where the hoist may be exposed to explosive gases, fibers, or dust.

                      - What is the altitude? Thin air at altitudes above 3100 ft is less efficient at keeping motors cool. Motors with cooling fans or larger frames may be required.

                      - Are there any other environmental considerations? For example, hoists used in the tropics may need protection against fungus, rodents, and insects that can damage electrical components.

                      • What are the lifting speeds or throughput requirements of the application?

                          • What is the duty cycle? This factor determines how heavy and rugged the hoist needs to be. A hoist that is not tough enough for the job provides poor service and wears out more quickly. Keep in mind that, according to ASME specs, the average load the hoist carries should be no more than 65% of its rated capacity.

                            • This duty cycle information should be provided:

                              - Number of lifts/hr

                              - Number of hoist motor starts/lift cycle required to raise, lower, and position a load

                              - Maximum weight of load handled

                              - Average weight of load handled

                              - Number of hours/shift hoist is used

                              - Shifts per day

                              - Days per week.

                              • Will grabs or spreader beams be suspended from the hoist? The addition of grabs or spreader beams can affect the hoist capacity-in effect, they are part of the load.

                                • - What is the weight of the grab or spreader?

                                  - Is it always on the hoist, or just used periodically? If the attachment is always there, it can hinder the performance of the hoist's mechanical load brake. A control and safety device regulates the lowering of the load. When manufacturers know these details, they can recommend different controls to avoid potential problems.

                                  - If the grab or spreader is used periodically, how often and for how long?

                                  - Is it electrically powered? If so, an electric cable reel to provide power may be necessary, as well as extra control buttons.

                                  • Will the hoist be used for bucket or magnet service? This kind of use also can affect the mechanical load brake. In addition, some applications may call for one hoist to hold the bucket and another to open and close it. Provide this information to the manufacturers.

                                    • - What are the weight, power, and control requirements for the equipment?

                                      - Is the magnet or bucket a hook-on or reeved-into type?

                                      - How often will it be used?

                                      - What capacity load will the bucket or magnet handle?

                                      • What special features are required?

                                        • - Is a special paint finish required? Besides environmental, other reasons such as visibility or safety are important.

                                          - Is an extremely slow hoisting speed needed for load positioning?

                                          - Are special hooks or a hook latch needed? A hook with a bullard latch, which is a heavy-duty safety latch, may be a good choice. It may be necessary to hook onto a special load, such as a sling or bale that requires a hook of a particular size or configuration to connect properly to an existing beam or grab. Or the application may require plate steel hooks for roll handling.

                                          - Are audio or visual warning devices beneficial? Horns, buzzers, and/or flashing lights can be added to warn workers when the hoist is operating or the crane is starting to move.

                                          • Besides improving speed, capacity, and/or controls, are there any problems with the performance of the current hoist that need to be solved or avoided?

                                              • Whether modernizing or keeping current electrification, provide details on the existing cross conductors that provide electrical power to the hoist and trolley.

                                                • - Are they festooned cable, shielded bar, or other form of enclosed conductor, angle iron, or bare copper wire?

                                                  - How many power and control conductors are in place?

                                                  - Where are they located in relation to the running surface for the hoist?

                                                  - What is the configuration and spacing of the conductors?

                                                  • What is the power supply?

                                                    • - How many volts are required?

                                                      - What is the frequency?

                                                      • Again, whether modernizing or not, it is important to answer these questions on controls.

                                                        • - Will the hoist be regulated from an existing control station?

                                                          - If so, is it a pushbutton pendant, radio control, or cab-mounted master switch?

                                                          -How many speed points are provided for control of each motion?

                                                          - Are these speed points of control sufficient for the new application?

                                                          -Edited by Ron Holzhauer, Managing Editor, 630-320-7139, rholzhauer@cahners.com

                                                          &HEADLINE>More info&/HEADLINE>

                                                          The author is willing to answer technical questions concerning this article. Mr. Miskowiec is available at 800-742-9269.

                                                          For more information on overhead handling systems, visit the " Material handling " channel at www.plantengineering.com.

                                                          &HEADLINE>Analysis of a crane system&/HEADLINE>

                                                          An overhead crane system is a specific configuration of equipment that makes it possible to move a load in three directions within a bay: up and down, left and right, and back and forth. The system is composed of three distinct parts: hoist, trolley, and crane bridge.

                                                          The hoist is a powered device that lifts and lowers heavy objects. It is attached to the second part of the system-a trolley-that travels left and right along the third component-a bridge girder or girders. Wheeled devices (end trucks) on each end of the bridge allow it to move back and forth along the length of the runway system. In fact, it's called a bridge because it bridges the distance between the columns of the building bay it operates in.

                                                          The crane system's ability to move a load in three directions enables an operator not just to lift loads up and down, but also to move them around throughout the bay. Without an overhead crane system, hoists are needed all over the place.

                                                          Trolleys and hoists can be separate pieces of equipment bolted together, or combined in a single built-up unit called a trolley hoist. Whichever approach is taken, it is useful to distinguish between the two types. A monorail or under-running hoist runs on the bottom flange of a single beam or girder. A top-running hoist runs on top of two girders positioned side-by-side.

                                                          It is also helpful to know the different types of cranes. Single-girder cranes can be top running or under running. Double-girder cranes are almost always top running.

                                                          Another useful term to understand is the envelope the hoist operates in. Essentially, this area is the clear space in which the hoist and trolley can travel unobstructed throughout the work area.





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