The Measurement Shop's Guide to Load Cells

Load cells are a fundamental component of all digital weighing scales and devices. The Measurement Shop’s comprehensive guide will provide crucial information on load cells, exploring load cell definitions, how they work, the different types of load cell available, and how to choose the best load cell for your needs.

What is a load cell?

A load cell is a transducer/ sensor used to convert a force acting on it into measurable electrical output. Load cells may also be known by the terms load transducers or load sensors. They are often used as part of weighing scales and balances. Load cells for scales are non-intrusive, highly-accurate measurements.

Who invented the load cell?

The origins of recognisable load cell devices date back to the mid-19^th century and were popularised by Sir Charles Wheatstone. Wheatstone popularised the ‘Wheatstone bridge’ by creating a bridge circuit that had the ability to measure electrical resistances. This device was used to measure resistance that occurred in strain gauges and became a frequent addition in laboratories.

In the mid-20^th century the bonded resistance wire strain gauge was developed and could be adapted with the Wheatstone bridge for use alongside mechanical scales or as stand-alone load cells and became a sensor used to measure static and dynamic strains. Due to their small size and ease-of-use, load cells have continued to develop and have now been adapted to measure many quantities related to strain such as pressure, force, acceleration and torque.

How do load cells work?

Different types of load cell measure output differently, according to the way in which they detect weight. Hydraulic load cells rely on pressure of internal filling fluid on the device in order to measure output. In contrast a strain gauge load cell converts force into measurable output via a gauge which is bonded onto beams or the load cell structure. When force is applied to the load cell, the structure will distort in relation to the weight placed on top of the device. This causes a change to the structure’s electrical resistance which can then be measured.

Types of load cells

There are many varieties of load cells available that have been specifically designed to measure electrical output in a number of ways in order to fulfil specific weighing needs. We have included a list below of some of the most popular types of load cells available:

Strain gauge

The most popular variety of load cell, strain gauge load cells consist of a spring mechanism on which a strain gauge has been placed.

Single point

Single point load cells, also called platform load cells, allow the construction of a weighing scale or machine with only one load cell. These load cells are commonly found in small to medium sized platform scales.

Pancake

Pancake load cells are low-profile compression and wheel tension load cells with a wide surface area for absorbing shock loads. Commonly used in heavy industrial, automotive and materials testing environments.

Shear beam

Shear beam load cells feature an oblong shape that usually include two retaining bolts at one end for added support and the load cell at the other end. Ideal for industrial environments and compression force testing applications.

S type

S type load cells, also called s beam or z beam load cells, feature a compact ‘s’ shaped design from which their name derives. These load cells are typically used for measuring suspended loads.

Compression

A compression load cell is a compact electrical device designed to hold a load at one point in order to measure compression. Compression load cells measure the pushing force exerted along a single axis.

Bending beam

One of the most popular load cells, bending beam load cells are compact, versatile devices that can easily fit into tight spots. These devices are commonly used in commercial measurement applications.

Double-ended

In contrast to regular shear beam load cells, the double-ended shear beam load cell has been secured at both ends, whilst the load is placed into the centre of the load cell.

Button

Button load cells are tiny, compression strain gauge measurement devices that work in areas with minimal space. The bottom of the load cell is often bolted, whilst the force is placed onto the button at the top.

Hydraulic

Hydraulic load cells measure weight as a change in pressure. When a load is placed onto the load cell, this can cause increased pressure that can then be measured by other electrical transducers.

How to replace/fit a load cell into a scale or balance

This process will differ depending on the scale or balance used, however the following will provide a general guideline on how to replace a load cell in a bench/ retail scale.

1. Ensure the power is switched off (both the mains and the scale).
2. Open the scale casing by carefully removing the screws in the designated areas.
3. Once the scale top has been removed you will see a variety of components. If replacing a load cell, it is common to find this fixed in position by an upper and lower metal frame. First, detach the load cell from the upper metal frame by unscrewing any central screws, then remove the upper metal frame by unscrewing the designated screws attached to the lower frame and carefully lifting.
4. Once the load cell has been exposed, detach the load cell by carefully disconnecting any wires. If the load cell, is screwed to the lower frame, you will also need to unscrew this prior to disconnecting any wires.
5. Once fully detached and removed, slot the new load cell onto the lower frame and reconnect any wires.
6. Carefully place the upper frame onto the load cell, ensuring the screw positions line up, and carefully screw in the central screw points to connect the load cell to the upper frame and then the exterior screws to fix the upper frame to the lower frame.
7. Once the load cell is secure, place the removed exterior casing back onto the scale and screw in to close.

Taking care of load cells

Load cells are sensitive measuring instruments that need to be regularly maintained in order to achieve the best possible results. Caring for your load cell can help to reduce maintenance costs, repair work and downtime, whilst improper handling of the device can cause premature malfunctions and damage to the load cell’s ability to provide precise measurements. Here we have provided a brief list of do’s and don’ts for maintaining your load cell:

 Do:

1. Choose the correct weighing capacity.
2. Ensure the environment is suitable for the product.
3. Ensure the load cell is calibrated regularly.
4. Check the storage and operating temperature.
5. Take care when installing your load cell.

 Don’t:

1. Don’t use your load cell over the stated weight capacity.
2. Don’t pull the load cell by attached cables.
3. Don’t over-tighten screws, bolts or other attachments.
4. Don’t use the load cell in an inappropriate environment (e.g. wet environments if the product does not have adequate IP-rated protection).
5. Don’t force the load cell into machines.

Cleaning your load cells

Weighing equipment needs to be regularly maintained and cleaned to prevent damage caused by dust and liquid ingress, a failure to do so can result in damage to the internal components of the machine and therefore pose a risk to the load cell mechanism inside. With this in mind, we recommend cleaning load cells regularly using a dry cloth to prevent the build-up of dust and materials around load cell connection points.

Most weighing scales are held together by a series of screw points which can be unscrewed using a screwdriver (larger machines may have been bolted and therefore require automatic drills to open); please refer to our earlier section on 'How to replace/fit a load cell into a scale or balance' for more information on how to detach your load cell for cleaning.

Load cell calibration

Load cells are an integral part of most weighing systems in industrial, aerospace and automotive industries, enduring rigorous daily use. Over time, load cells will drift, age and misalign therefore they will need to be calibrated regularly to ensure accurate results are maintained. As a general rule, most load cells users opt for a re-calibration period of 1 year, dependent on the level of load cell deterioration.

Standard calibration tests will use linearity and repeatability as a calibration guideline as these are both used to determine accuracy. Calibration is conducted incrementally starting working in ascending or descending order. For example, if you have a 60 tonne load cell, then you way use specific test weights that measure in 5, 10, 20, 40 and 60 tonne increments – A five step calibration process is usually sufficient for ensuring your device is accurately calibrated.

Most load cell manufacturers will provide this service, however we recommend contacting them directly if you require more specific information about the calibration process and certification.

Buying a load cell

When buying a load cell, you will need to consider how you intend to use the device. This brief list will highlight some of main things to look for before purchasing a load cell.

Type

The first thing to consider is the type of load cell you need. Load cells come in many shapes and sizes and have different capabilities that make them more or less suitable for specific applications. Before buying a load cell, we recommend researching which types of load cell are commonly used in your field.

Cost

Load cells can prove to be expensive devices. We recommend setting a budget on how much you wish to spend when buying a load cell in regards to the features you require.

Intention

How you intend to use the load cell can make a huge difference on your selection. For example, if you are looking for a load cell to go with your hanging scale for lifting products, than an s type load cell may prove better than a compression load cell commonly used for vehicle scales.

Features

Different load cells come with different features that may be more or less suited to your industry. If you work in wet areas, you would need a waterproof IP68-rated load cell for best results. If you are looking for a load cell suitable for outdoor use, then an IP68-rated load cell with advanced lightening protection may be required.

Capacity

You will need to make sure that the weight capacity of the load cell is suitable for the products you wish to weigh. If you are looking to weigh heavy containers over 20 tonnes then you will need to ensure that your load cell is capable of weighing loads from 20 tonnes and higher.

TMS range of load cells

BM14C compression load cells

Zemic’s BM14C is a stainless steel, IP68 rated compression load cell with welded sealing. This load cell offers significant lightening protection that can reach 10,000V with automatic recovery. Ideal for use in automotive engineering and outdoor environments alongside vehicle weighing scales.

L6Q single point load cells

The Zemic L6G single point load cell is a sleek IP65-rated anodised aluminium device with OIML Class III approvals. These load cells are ideally suited to price computing scales, retail scales and shop scales.

H8C shear beam load cells

OIML approved shear beam load cell for total protection against liquid ingress. The H8C is a highly popular, versatile load cell constructed from nickel-plated steel alloy. Ideal for multi-purpose measurement alongside truck scales, track scales, vessels and hopper scales.

B3G S type load cells

Zemic’s B3G S type load cell features a compact, durable design perfect for hanging scales and force measurement devices. B3G load cells include IP68-rated, stainless steel construction and are Class III OIML approved and sealed for tension and compression applications.

H9C double-ended shear beam load cells

Highly-stable, dual-ended shear beam load cell constructed from IP67-rated alloy steel. This high-performance device is perfectly suited for use in the aerospace and automotive industry alongside vehicle scales, track scales and automobile testers.

BM11 bending beam load cells

Versatile, IP68-rated waterproof bending beam load cell manufactured by Zemic. The BM11 is hermetically sealed to prevent damage to the internal components of the device. Perfect for use alongside industrial pallet scales and platform weighing scales.