Maintenance and Inspection of Below-the-Hook Lifting Equipment

Many of the C-hooks, coil grabs, tongs, and load beams used in the steel industry are among the least expensive components of the steel processing line. However, when the shipping dock stops loading coils because a coil grab ceased operation, or the furnace shuts down due to a cracked bushing in the linkage of the ingot tong, the equipment's cost takes on a new perspective. The purpose of this paper is to map out better maintenance and inspection techniques to help prevent inadvertent downtime of the line due to breakdowns of "Below-the-Hook" material handling equipment.

C-hooks
During Frequent Inspections, our inspectors first do a preliminary visual inspection of the entire C-hook to look for obvious problem areas: bail wear, bent bails, bent lower arm.  They also ensure that the manufacturer’s nameplate and safety labels are attached. The next step would be to concentrate on visually looking for cracks at the crotch of the c-hook and verifying that the coil's inside edges are not gouging out metal from the crotch. One way to visually discover cracks is to look for cracking paint at the crotch that is not evident at other parts of the C-hook; many times the base metal will yield more than the hardened layer of paint on the surface. If there is any indication of contact between the coil and the crotch of the C-hook, a change in coil handling procedures is warranted. Continued contact of metal on metal at the crotch can create a site for crack initiation. The basic design of a C-hook creates a cyclic loading of the lifter, with the maximum load being concentrated at the crotch of the C-hook. These conditions set up the crotch of the C-hook as a potential site for crack propagation and subsequent material failure.

The next inspection would be to verify that the lower arm has not been bent so that it is out of parallel with the C-hook, or bent down as a result of excessive load on the lower arm. To determine if the lower arm is parallel with the rest of the assembly, we use a long straight edge and allow one to two percent of variance due to original manufacturing of the plate steel. To detect excessive loading on the C-hook, the inspector can measure the inside dimension between the upper part of the C-hook and the lower arm. More than a 1/2-inch variance would be considered unacceptable.

The inspector should look for bail wear, gouging, and other discrepancies as previously discussed. A careful visual inspection of the welds that hold the bail to the base metal would be performed. The welds that hold the counterweights onto the C-hook will then be inspected; these are not usually structural welds, but failure of these welds could be hazardous to operators and equipment in the vicinity.

Inspection of the saddle that the steel coil rests on is important to the economic handling of the coil, but is usually not a structural concern. Pads that may be applied on the nose of the C-hook or on the back vertical riser, revolving belts or other assemblies that protect the coil should be inspected. Each company should determine a quantifiable set point forallowable degradation of the protective surfaces that their maintenance department can use to determine when replacement is required.

During Periodic Inspections, the same inspections as described above are performed and recorded for trending data. In addition, the following items are performed:

• Dye-penetrant checks are performed at the lower and upper crotch of the C-hook. After removing paint, oil, and other debris, this non-destructive test should indicate no cracks in the base metal.
• Dye-penetrant checks are performed at all welds on the bail assembly (and bail pin if applicable.) After removing paint, oil, and other debris, this non-destructive test should indicate no cracks in the welds (or in the base metal of the pin.)

Motorized Coil Lifters
During Frequent Inspections, our inspectors first do a preliminary visual inspection of the entire coil lifter to look for obvious problem areas: bail wear, smooth operation of the sliders, smooth rotation of the grab (if applicable,) all guards and stops are attached, and that the manufacturer's nameplate and safety labels are attached. Our next step is to check each of the safety features associated with the lifter. (A safety feature is defined as an interlock that prevents loss of control of the load.) Many coil lifters have a device that detects when a coil is engaged and simultaneously disables the ability of the grab to open the sliders. These safety features should be challenged without a load and verified that they work properly. One method is to compress the pad switches and attempt to open the grab; it should not open. If there are any indications that this safety feature does not work or has been tampered with, the lifter should be immediately tagged out of service until repairs can be made. Other interlocks that protect the coil should be checked including flappers or proximity sensors that prevent damaging the coils when the sliders are closed. Failure of these interlocks needs to be addressed individually, based on their relative importance to the protection of the coil.

The structural components of the lifter can then be inspected. A visual inspection of the slider would include verifying the wear surfaces are properly greased and have no excessive gouges. The welds at the "knee" of the slider (the 90° turn when the slider becomes the vertical leg) should be checked for obvious cracks. The horizontal pad that the coil rests upon during handling should be checked for cracks at the crotch of the leg. Visual inspection of the welds that make up the attachment point between the lifter and the crane is necessary. Ensure the keeper bar for the bail pin is attached and the retaining devices are in working order. The bail pin or bail should be visually inspected as discussed in the previous section. Any weld cracks  that are considered hazardous to the structural integrity of the lifter should warrant the lifter being tagged out of service until repairs can be made.

Before the electrical inspection begins, all power should be disconnected at the source per ANSI Z244.1 "Personnel Protection Lockout/Tagout of Energy Sources" and the lifter verified as de-energized with a multi-meter. The first thing we check in our electrical inspection is the plug and cabling between the crane and the lifter. The plug and receptacle housings should have no cracks, and there should be no signs of wires sticking out of the plug or receptacle. Cord grips at the plug/receptacle and at the junction box on the grab should be installed to take all of the strain that the ends of the cable could be exposed to. Conversely, the pendent (if applicable) should be inspected and verified to ensure that the cable is not frayed, has proper cord grips, and the enclosure is not cracked or broken. A visual inspection inside the electrical controls enclosure should reveal no indications of burned or cracked wire insulation, foreign material lying at the bottom of the panel or obvious loose connections. Outside of the enclosure, the wiring to the sensors and motors should be inspected. Discrepancies to look for include frayed wires, sliced insulation, stretched or  taut wires, discolored insulation or broken connectors. Any electrical discrepancy that is considered hazardous or violates the NFPA-70 National Electric Code should warrant the lifter being tagged out of service until repairs can be made. Once the lifter is re-energized, the pendent should be tested through all of its operations to verify that all controls function properly.

The mechanical drive and rotation (if applicable) components should be inspected for wear and alignment. Verify the integrity of the bearings, chains, belts, couplings, clutches and other drive line components. Is there an obvious alignment problem or wear indications that would translate into failure?  

Are there any foreign materials or metal shavings in the vicinity of the drive components? Are the chains and belts properly adjusted? Check to see that greasing and lubrication of these moving components have been properly performed. Operate the drive system with the covers off and listen and watch for noises or irregularities that could indicate potential problems.

During Periodic Inspections, the same inspections as above are performed and recorded for trending data. In addition, the following items are performed:

• Dye-penetrant checks are performed at the crotch of the horizontal pad that the coil rests on. After removing paint, oil, and other debris, this non-destructive test should indicate no cracks in the base metal.
• Dye-penetrant checks are performed at all structural welds in the bail assembly and the knee of the sliders. After removing paint, oil, and other debris, this non-destructive test should indicate no cracks in the welds (or in the base metal of the pin.)
• Record the alignment of the bull gears in the drive components. Significant changes over time in angles between the bull gear and other drive components such as the gear rack may indicate unacceptable wear or potential bearing failure.
• Record the clear height (the distance from the horizontal pad that the coil rests on to the bottom of the lifter frame) as the sliders extend. Measure the clear height with the sliders closed and then when open. Most models experience a difference in the clear height in these two positions when the slider droops at their furthest extension. However, if this clear height sharply increases over time or the sliders hang up when fully extended, there is indication that the wear surfaces have excessive wear or other more serious structural problems may be occurring.
• Do the legs operate simultaneously? If your coil grab has a drive and idler side configuration, measure the difference between when the drive side starts movement and when the idler actuates. We use a pair of dial indicators to measure the difference in movement. If this difference sharply increases over time, bearing, chain, or belt failure may be indicated.

Some recommend maintenance practices include:

• Coat teeth of rack and pinion with a product such as jet lube gear guard open gear lubricant.
• Lubricate sliders and slide ways using a product such as Benz oil moly alumaplex EP #2.
• Lubricate bearings and/or bushings as needed.
• For rotating coil lifters, inspect and lubricate the ring bearing per manufacturer instructions.
• Lubricate roller chains, bull gears, sprockets, etc. as needed.
• Many reducers have vent plugs that need to be cleaned per manufacturer instructions. (Verify that when the lifter was received at the plant, a solid-pipe-plug was not left in after the lifter was put into service at the plant.)
• Using a test load, adjust the clutch per manufacturer's instructions.
• Check the reducer or gear motor for proper oil levels. If the level is low, add proper lubrication through the filler plug until it comes out of the oil level plug; refer to the manufacturer's manual for the location of each plug.
• Are the reducer seals leaking? These seals are wear items and need replacing periodically.
• Replace wear pads or other steel coil protective options as needed.
• Adjust the chain or belt slack. We allow for approximately 2% slack.

Please consult with the coil lifter manufacturer for specific maintenance instructions and lubricants.

Tong Grabs
During Frequent Inspections, our inspectors first do a preliminary visual inspection of the entire tong grab to look for obvious problem areas; bail wear, smooth operation of the linkages and safety latch, all guards and stops are attached, and that the manufacturers nameplate and safety labels are attached. Then the following components are reviewed:

• Inspect the bail and determine the loss of material that has occurred where the tong interfaces with the hook. (See previous section on Bail Wear.)
• Inspect the pins that connect the linkage of the tong. Are there wear indications? If there is more than 2 - 5% (obvious indentations in the pins), consult the manufacturer about replacement.
• Inspect the automatic latch. Ensure that there is no peening or wear at the interface between the piston and the catch.
• Are the pins that connect the linkage straight and round? We tend to see deformation of the pins before the links show signs of overloading.
• Are the retaining devices that hold the pins in place (collars, roll pins, cotter pins, etc.) intact and working properly?
• Are the bushings in good condition, or are they cracked and worn? Any indications of cracks should result in the lifter being tagged out of service until repairs can be performed.
• How much play or "slop" is in the linkage? This may indicate the retaining devices are worn or the bushings/bearings are worn.
• Are the legs of the tong straight, and do they meet in the center when the tong is closed? Bent linkages could indicate that the tong has experienced excessive loadings that have caused permanent yielding of the base metal (metal deformation.) These linkages should be replaced.
• Have any unauthorized modifications been performed on the tong?
• Are there any visual cracks of the linkages?
• Check fasteners, covers and stops to ensure they are properly attached.
• Operate the grab and verify that it works smoothly. Does the automatic (manual) latch work properly?
• Do the pads operate properly (swivel, rotate, if applicable)?
• Are the pads or points worn to the extent that they need replacement? It is important that these interface points are maintained since the gripping ratio or coefficient of friction that they create may be a basis of the original design. Contact the lifter manufacturer for allowable wear of these components.

Periodic Inspections would include documenting the same inspections described for the Frequent Inspections plus:

• Dye-penetrant testing of the bail assembly. No crack indication is the criteria.
• Dye-penetrant testing of the pins that hold the linkages together. "No crack observed" is the minimum criteria that we find acceptable.

Lifting Beams 
Lifting beams come in many different configurations, but the inspection of them is very similar. The Frequent Inspection starts with a global review of the beam which looks for obvious material deformation, bent hooks, missing retaining pins, keeper bars, safety signs and manufacturer's labels. We would then inspect the hooks or attachment points of the load to the beam:

• Are the hooks bent? If so, they need to be replaced.
• Are the pins that connect the "J" hooks or other lifting points to the beam in good condition? If there is more than 2 - 5% (obvious indentations), consult the manufacturer about replacement.
• For beams with adjustable lifting points or bails, we carefully inspect the mechanism that holds the assembly in position. Is there sufficient wear or degradation that would allow the assembly to inadvertently slip out of position during a pick? Are the pins or clips that hold the position in good condition?
• Are the shackles/links/hooks/slings in proper condition? Do they have the pins or other retaining devices to prevent the load from being released? Are they sized properly if they have been substituted from the original design?

Visually inspecting the lifting beam for obvious weld cracks or other signs of deformation would be the next step. Cracks in structural members would warrant the beam being tagged out of service until repairs can be made. Cracks in spacers or other non-load bearing members need to be evaluated to determine if they would be detrimental to the operation of the beam. The bail or bail pin between the crane and the beam should be inspected for obvious wear and excessive indentations as discussed in the Bail Wear section.

If the lifting beam is made of channel, I-beams or other structural members, checking the straightness of the beam may indicate if the beam has been subjected to excessive forces or loads. A simple piece of string pulled taught along the edge of the structural member will quickly determine the difference in the camber and sweep of the lifter. Anything in excess of 3° out of alignment should be investigated.

During Periodic Inspections, the same inspections as above are performed and recorded for trending data. In addition, the following items are performed:

• Dye-penetrant checks are performed at the critical loading areas on all hooks or other members that connect the load to the beam. After removing paint, oil, and other debris, this non-destructive test should indicate no cracks in the base metal.
• Dye-penetrant checks are performed at all structural welds in the bail assembly and the beam. After removing paint, oil, and other debris, this non-destructive test should indicate no cracks in the welds (or in the base metal of the pin.)

Maintenance of a lifting beam is usually limited to replacement of protective pads, liners or hardware that attaches the load to the beam. On beams with adjustable bails or hooks, properly greasing the contact points would be advisable.

Note that many lifting beams are motorized for rotation or other axis motions. If you have a motorized unit, many of the items covered in the motorized coil lifter section will pertain in this case including electrical safety, bearings, gears, clutches, reducers, etc.

Conclusion
The mill duty equipment found in the industry is usually designed for a severe duty cycle and minimal maintenance. The inspection criteria and maintenance procedures in this paper are what we at Bushman Equipment have found to be useful in maximizing the longevity of the lifting equipment. This is not intended to usurp the original manufacturer's recommendations or other regulatory authority. Although required by ASME standards, inspection of all lifting equipment is also a prudent maintenance procedure because it will improve the overall productivity of the manufacturing line. Maintaining a regular inspection and maintenance program on the lifters will help ensure a long useful life of the lifter and a better return on your investment.

Reference
1 ASME B30.20 "Below-the-Hook Lifting Devices."

Accurate Load Weighing During Coil Handling

Accurate coil weight is becoming increasingly important as customers and manufacturers work to improve the efficiency of their process lines and facilities. A true coil weight is used in a number of different applications including determination of price, line efficiency, and transportation cost.

To obtain this weight, manufacturers and suppliers have used a variety of methods that normally require the coil to be handled multiple times as it is loaded and unloaded from the scale pads. This method of handling reduces the efficiency of the process line while significantly escalating the potential for coil damage. Another method of obtaining the accurate weight of the coil is to integrate the load weighing apparatus into the crane equipment, specifically the "Below-the-Hook" lifters.

Currently, the most common method of determining weight is using the strain gauge. There are only a few different types of load cells that incorporate strain gauges and are practical for use in "Below-the-Hook" applications. They include the canister, single ended shear beam, double ended shear beam, cantilever beam, and the "S" beam. The canister is usually a hermetically sealed assembly that can be purchased as a tension, compression or universal cell. In the tension application, the mounting device is threaded into the cell, which allows for a positive attachment point when "stretching" the strain gauge. Conversely, in the compression application, the force is applied to a load button or other raised surface on the cell. Although these are the earliest load cell designs, they tend to be more costly than other types and do not work well with side loading when the forces are not perpendicular to the cell.

Shear beams (single ended and double ended) are manufactured with the strain gauges embedded in a thin membrane inside the cell. These cells are used in low profile assemblies; they come in hundreds of different styles, and are very cost effective. The single ended shear beam is bolted at one end with multiple fasteners and the load is applied at the other end. Double-ended beams are bolted at each end and loaded in the center. Double-ended beams are commonly used in bail pin applications for coil grabs and C-hooks (Figures 2 and 3.) Cantilever beams are similar to shear beams, but instead of mounting the strain gauges on a thin membrane, the beam is machined all the way through, and the gauges are mounted inside the beam along the machined inner edges. Specialty load cells are commonly made in this configuration; an example is shown in Figure 4, a rotating bottom block. The last load cell is the "S" cell, which gets its name from its shape. "S" beams are commonly used in tension applications and are excellent when cables are used in the assembly.

There are an infinite number of ways to mount the load cell on "Below-the-Hook" lifting devices. One of the easiest methods is to put the cell in series between the crane hook and the lifting device. There are many commercially available devices on the market that can provide multiple features with regard to power, display units and controls. Figure 1 shows an MSI-4260 from Measurement Systems International attached to a c-hook. The benefits of these scales are that they are readily available and relatively inexpensive. One of the biggest drawbacks is that for large capacities, they will extend the head height of the load by four to five feet.

To reduce headroom, the load cell is often incorporated into the lifting device. Figure 2 shows a "C" hook in which the load cell is built right into the bail pin using a double-ended shear beam style load cell. The crane hook rides on a spool piece that slips around the load cell, making sure that the hook remains centered on the cell. Figure 3 shows a coil grab that uses a canister style load cell to determine the weight of the load and thus protect the cell from the hook.

There is no design book available that tells you how to design a "Below-the-Hook" lifting device. Likewise, there is no template that tells you how to solve all your load weighing problems for lifters. However, based on our experience, we use the following guidelines and important considerations when we design load cells into lifters:

1.         The load cell capacity must be selected based on the capacity of the lifter (live load), the weight of the lifter that is below the load cell (dead load) and all fittings, slings, chains, etc. All weight that the load cell will be subjected to must be included in the load cell capacity calculation, not just the live load. When sizing the cell, also consider the type of shock loading that will be applied. Most cells have a normal safe overload of 150% of full-scale (F.S.) or more. If your cell is on a production line in a mill or other heavy-duty application where the load cells will have significant shock loads applied, this overload could be challenged frequently. You may want to consider increasing the capacity of the load cell to allow for this service factor.

2.         The ASME B30.20 "Below-the-Hook" Lifting Devices standard governs structural design of a lifter and requires that a minimum of a 3:1 safety factor based on yield is maintained. Many load cell manufacturers design to a 4:1 factor of safety based on ultimate strength, which can be less stringent. Make sure you understand the cell manufacturer's design criteria and adjust to meet the B30.20 requirements.

3.         The load cell should be designed so that it can account for the entire load. Sliding mechanisms or guide bars which align the lifter can absorb some of the load or can cause friction which would introduce errors in the load readings.

4.         The load cell must be loaded in accordance with the design criteria of the manufacturer. Excessive side loading or unequal loading can introduce inaccuracies or damage the load cell.

5.         The attachment points of the load cell hardware assembly must be aligned properly, and the assembly should be aligned vertically. Side loading of certain cells will introduce inaccuracies into the load reading. For consistency, consider using spool pieces or other devices to keep the hook or attachment points centered and in the same position during each lift, even with different operators. If the hook can "walk" on a load pin, the angle of force on the load cell may change slightly, resulting in a change in the output of the strain gauges.

6.         The load cell should be designed to be easily removed from the assembly for calibration and/or replacement. The current commercially available models tend to be bolt-in designs that facilitate this type of installation. If you are using a compression style load cell, the striker plates also need to be designed to be removable. The striker plate contacts the load button of the canister and applies the load to the cell. Over years of use, the point where the striker plate contacts the load cell can become worn and cupped. This irregular shape can cause side loading on the load cell, which may result in discrepancies in the reading or damage to the cell.

7.         Power to the load cell must be considered. Many commercial load cells come with battery powered displays. (Figure 1 is an example.) The positive feature is that this eliminates the need for cables and cable reels on the crane. The negative feature is that you may be replacing batteries frequently, and the limited power supply may eliminate the ability to have a number of useful features such as large displays (scoreboards), modems, and anti-sway programming. If electrical power is brought to the load cell, this power supply should be protected and continuous. Protected power means using high quality surge protection and/or a Universal Power Supply. Continuous power is defined in this context as not having the power routed through the lifter control system and cycled when the lifter is opened and closed.

When considering the design and type of load cell, the engineer will need to determine if he wants to use a "certified" load cell and if the application needs to be "Legal-for-Trade." "Legal-for- Trade" is an ambiguous term that broadly says that the load cell and equipment conforms to Handbook 44, "Specifications, Tolerances, and other Technical Requirements for Weighing and Measuring Devices." Handbook 44 is not a federal law, but it carries the same weight. It is a set of comprehensive requirements for weighing and measuring devices that are supported and published by the government and adopted by all 50 states. If the load cell application that you are designing will be used in commerce (selling material based on weight), then the load cell should conform to Handbook 44.

Verifying that a load cell conforms to Handbook 44 is an enormous task and would be insurmountable for most designers. Instead, the designer should look for load cells that have an "NTEP" certification. The National Type Evaluation Program (NTEP) is a nation-wide program with an approved process for testing, examining, and evaluating weighing equipment to ensure its compliance with the provisions of Handbook 44. Load cell manufacturers submit the necessary documentation, load cell samples and payments to have a third party laboratory test the load cell and issue a Certificate of Conformance (C of C) for the load cell. (Note: NTEP is a voluntary program that is not required by all states; however approximately 37 states require a C of C for devices used in commercial weighing applications within their jurisdiction.) The NTEP program is useful to the designer because it provides a bench mark to compare different load cells. However, it does not eliminate the need for a quality design for mounting the load cell or the requirement for periodic calibration of the system once it is installed in the field.

One of the biggest problems with "Below-the-Hook" load weighing is the swing of the load as the crane raises, lowers or traverses with the load. There are a number of different products on the market for minimizing the effect of swing on the load readings being displayed to the operator. They use a combination of electronic filtering to reduce the effect of normal crane movement on the load output. Some of the techniques used include:

• Analog filtering which helps smooth the shape of the incoming load cell signal by eliminating electrical noise or interference that is riding on top of the DC signal provided by the load cell. By removing the higher frequency interference components, analog filtering aids in lessening the influence of this noise and interference.
• Digital filtering averages weight readings mathematically to minimize the effect of any bumps seen by the indicator. It works by averaging a pre-determined number of readings and outputting the average of these settings. This can increase the settling time, but does not affect the analog-to-digital (A/D) converter measurement rate, display update or output rates.
• Display filtering is a feature that can be adjusted so the update rate of the display is reduced to minimize the rapid turnover of the scale, but it does not affect the actual A/D converter. This allows for easy screen viewing and does not affect the accuracy of the system.