In sheet metal processing industries, accurate coil weighing 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 transport cost.
To obtain this weight, manufacturers and suppliers have used a variety of methods that normally require the coil to be handled several times as it is loaded and unloaded from the scale pads. Process line efficiency is reduced and the potential for coil damage is increased. Another method of accurately obtaining coil weight is to integrate the load weighing apparatus with the below-the-hook coil lifter.
The most common method of determining weight is to use a strain gauge. Only a few types of load cell incorporating strain gauges 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 available 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, come in many different styles, and are 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 centre. Double-ended beams are often used in bail pin applications for coil grabs and C hooks 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. Speciality load cells are commonly made in this configuration; an example, shown on p29 in the lower picture, is a rotating bottom block. ‘S’ cells get their name from their shape and are used in ‘S’ beams, commonly in tension applications and are particularly suitable when cables are used in the assembly.
There is a myriad of ways to mount the load cell on below-the-hook lifting devices. One of the easiest is to put the cell in series between the crane hook and the lifting device. Large numbers of devices are available offering combinations of many features for power, display, units and controls. 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 up to 1.5m.
Design guidelines
There is no design book available that details how to design a below-the-hook lifting device. Likewise, there is no template showing how to solve all load weighing problems for lifters. Based on experience, however, the following guidelines and important considerations can be used for designing load cells to use in 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, the type of shock loading that will be applied should also be considered. Most cells have a normal safe overload of 150% of full-scale (F.S.) or more. If the cell is on a production line in a mill or other heavy-duty application where the load cells will be subjected to significant shock loads, this overload could be challenged frequently. It could be that load cell capacity should be increased to allow for this service factor.
2. In the USA the ASME B30.20 standard, Below-the-Hook Lifting Devices, 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 all of the 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 centred 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 might 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 a compression style load cell is used, 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 can mean discrepancies in the reading or cell damage.
7. Power to the load cell must be considered. Many commercial load cells come with battery powered displays. A positive feature is that this eliminates the need for cables and cable reels on the crane. A negative point is that you might 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.
Legal-for-trade
When considering the design and type of load cell in the USA it needs to be determined if a certified load cell is to be used 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 is being designed 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 in the USA should look for load cells that have an NTEP certification. The National Type Evaluation Program (NTEP) is a US-wide programme with an approved process for testing, examining, and evaluating weighing equipment to ensure 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 on the load cell and issue a Certificate of Conformance (C of C). NTEP is a voluntary programme that is not required by all US 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. It does not, however, 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.
Minimising sway
One of the biggest problems with below-the-hook load weighing is the swing of the load as the crane raises, lowers or traverses. There are a number of different products on the market for minimising 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:
• An analog filter that 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 helps to reduce the influence of this noise and interference.
• Digital filtering averages weight readings mathematically to minimise 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 minimise 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.
• When adjusting the electronic filtering capabilities, make sure that you do not negatively affect the output of the processor to an extent that invalidates the system for use in a legal-for-trade application.
Transmitting data
When an accurate load reading has been established from the processor, the data needs to be transmitted in a useable form. The standard method to display the weight is with a digital display. Displays are widely available off the shelf with characters up to 100mm high. Mechanical flip-number displays are sometimes used, but they should only be on stationary fixtures with the load data transmitted via a modem to the remote scoreboard.
The load data can also be used to support the maintenance department and to help reduce downtime. An example is where crane cables, lifters, bottom blocks and trolleys are frequently damaged when an operator picks up a load that exceeds the crane capacity, or a coil that is still attached to a rail car. By using one of the data export connections on the load cell processor, a signal can be sent to a relay in series with the crane hoist control that prohibits further up-motion if the load’s weight exceeds a pre-determined set point.
With today’s business environment requiring data to be available instantaneously to a wide spectrum of users, alternative methods need to be employed to disseminate the data. Most load cell processors can be connected to a computer network by a cable or a modem. This data can be exported into an unlimited number of ERP/MRP systems for input into shipping reports, bills of loading, automatically printed tags, production efficiency reports, etc.
If the load of each coil needs to be tightly controlled, the information can be fed back into a process control system to change the necessary coil parameter to achieve the desired performance. In addition, this electronic data can be transmitted via the internet to the customer in the form of advanced shipping notices or faster invoices.
Sheet metal companies regularly require accurate load weighing that does not negatively affect delivery schedules or increase damage to the coils. As more and more companies standardise their processes under quality programmes and require additional data on their purchases, this trend is only going to increase.
Manufacturers need to be flexible to adapt the latest technology into their processes and to provide the additional services that customers demand. If the technology is incorporated into the design of the lifter from the beginning then quality load weighing data can be achieved and transmitted to a multitude of users.
