DESIGN AND SELECTION
OF TROLLEYS Hand-pushed trolleys are used to control manual handling risks because they provide an alternative to carrying loads by hand. Trolleys can also help us move larger loads. However, under some circumstances, the forces needed to manoeuvre a loaded trolley can introduce a significant manual handling risk. The way in which you load and unload trolleys can also create a risk of manual handling injury. Appropriate selection or design of trolleys is essential to ensure that you can use them effectively and safely. The task, the workers and the work environment also contribute to the safety and effectiveness of the trolleys. In some cases, you will have to change the task or the work environment to use trolleys safely. These trolley guidelines focus on reducing manual handling risks but injuries also result from slips, collisions, trolleys tipping over, crushing between trolleys and walls, and trolley wheels rolling over feet. Improvements in trolley design, work procedures and environment can also help to reduce all of these injuries. These guidelines apply to standard and special purpose hand-pushed trolleys that can be used in a range of industries, including manufacturing, retail, hospitality and health. Most of the recommendations are taken from a research report by Lawson and Potiki (1994)11 that provides detailed guidelines for trolleys intended for the health industry.
THE DESIGN AND SELECTION PROCESS Many examples are found in industry where a trolley has been purchased but remains unused because it is inappropriate for the task or environment. In other examples workers are constantly exposed to unnecessary risk of injury because of inappropriate trolley design. Poor maintenance is another reason trolleys are either unused or unsafe. In these cases the workers who use the trolleys usually have a good understanding of what is wrong with the trolley or the task they are expected to do with it. Therefore by consulting with workers you can minimise the risk of a new purchase being either unusable or an ongoing risk to health and safety.
1
Lawson, J and Potiki, J. (1994) Research Report: Development of Ergonomic Guidlelines for Manually-Handled Trolleys in the
Health Industry (Unpublished). The research was sponsored by the Central Sydney Area Health Service and funded by a grant from Worksafe Australia.
It is not just common sense to consult workers when choosing a trolley. The OHS Act and Regulations make consultation with workers (or their representatives on health and safety issues) compulsory when manual handling risks are being assessed and when risk control measures are introduced. Before you decide that a trolley is the right solution to a manual handling problem it’s important to consider all options eg you may find ways of changing a process to eliminate the need for moving the load at all. You may also be able to use other mechanical means of moving things, such as a forklift or conveyor. If you decide that a hand-pushed trolley is required, the following steps should be taken when preparing a specification for the design or purchase: a. Consult with workers to find out what the problems are with the existing way the task is done and to provide information about some of the options available. b. Assess the task for which the trolley will be used. Consider all the risk factors in Part 4.4 of the OHS Regulation 2001. Measure or estimate weights to be handled and distances to be moved. If trolleys are currently in use, measure push/ pull forces along the routes travelled. A simple spring balance is all that is required to do this. c.
Involve an OHS professional in preparing the specifications for new equipment.
d. Get advice about trolley types and options from a trolley supplier. e. Wherever possible, try out new equipment before purchase. Perhaps you can build a prototype or mock-up of new equipment that is being designed and have it tested on the job before finalising the specifications, or you can borrow an existing trolley of the same type. f.
Consider how the trolley will be maintained, who will be responsible for maintenance and how frequently the trolley will be inspected.
g. Develop a procedure for the safe use of the trolley that includes load limits, areas where the trolley can be used and training of the workers.
SELECTION FROM STANDARD TROLLEY TYPES In many situations a trolley can be purchased or adapted from a standard range. The most common types are discussed below. Many other types of trolleys are available for specific purposes.
Two-wheel trolleys Two-wheel trolleys or ‘hand trucks’ are versatile and widely used on delivery vehicles and for moving furniture. They are good for: ► tall items that must be picked up and set down at floor level ► delivery work, where the trolley itself is frequently lifted on and off the vehicle ► uneven terrain and over gutters.
Two-wheel trolleys require the user to support some of the load. For this reason they are not the best option for long distances on smooth floors where a platform trolley could be used.
Low-platform trolleys Low-platform trolleys are stable and suitable for luggage and large cartons. The low platform means that a large volume of stock can be carried without the overall height becoming excessive. However, you have to bend down to pick up the lowest items unless the items have handles at the top.
High-platform (traymobile) trolleys The top deck of a high-platform trolley keeps items at a more convenient height for manual handling. Lower levels provide extra capacity but should not be used for heavy or awkward items.
Box-sided trolleys Basically a box or tub on wheels, these are used for linen and loose items that are not easily stacked on a platform. A disadvantage is that it is often necessary to bend down over the sides to get items out (opening sides or spring-assisted lift inserts can overcome this problem).
DESIGN GUIDELINES: ► Push/pull forces The push/ pull forces required to move a trolley must be kept within safe limits, in order to reduce the risk of strain injuries. Reducing the force needed also reduces the risk of slipping while trying to move and control a trolley. The initial force required to start a trolley is usually higher than the force needed to keep it moving in a straight line, especially if the swivel castors are not already facing the right direction. Lawson and Potiki recommend maximum push/pull forces in the range of 17-21 kg for initial force, and 6-12 kg for sustained force. The values at the lowest end of the ranges given above are recommended, particularly for tasks done frequently or for a long time. Values at the highest end of the ranges are maximum limits for infrequent, short duration tasks. High peak forces are sometimes encountered when pushing trolleys across a short ramp or over an irregularity in a floor surface. These peak forces should not exceed the specified initial force limits. Where a trolley is pushed over several different floor surfaces, the recommended sustained force limit should not be exceeded on the most resistant floor surface. (The force required to push a trolley on carpet is typically 30 per cent to 50 per cent higher than on a smooth hard surface). If trolleys are pushed up a slope (unless it is only for a very short distance), the load should be limited so that the recommended sustained force limit is not exceeded on the slope. Table 1 shows calculated pushing force increases per 100 kg of laden trolley weight on various slopes. These must be added to the pushing force needed on a level surface.
Table 1: Increase in pushing force on various slopes Slope gradient
Push force increase per 100 kg of laden trolley weight
1 in 10 (5.7°)
10.0 kg
1 in 15 (3.8°)
6.7 kg
1 in 20 (2.9°)
5.0 kg
1 in 30 (1.9°)
3.3 kg
1 in 50 (1.1°)
2.0 kg
For example, if a trolley with a laden weight of 150 kg requires a pushing force of 3 kg on a level surface, it will require a force of 10.5 kg (3 + 5 (150 °÷ 100)) to push it up a gradient of 1 in 20. The push/pull forces can be reduced by: ► reducing the total weight of the loaded trolley ► replacing carpets with smooth, hard floor surfaces ► using wheels with a lower rolling resistance eg larger diameter, harder tyres, better bearings. If the push/pull forces cannot be kept within the recommended limits, then consider getting assistance from another person or using a mechanical towing device. ► Types of wheels The choice of wheels and castors has a significant impact on the safety and ease of handling of a trolley. The factors that must be considered in order to minimise rolling resistance are discussed below. Wheel diameter Larger wheels have lower rolling resistance than smaller ones and are less affected by gaps, ridges and irregularities in the floor surface. A minimum diameter of 200 mm was recommended by Lawson and Potiki for all trolleys that have a laden weight over 200 kg or that are used outdoors. For other trolleys a minimum diameter of 125 mm is recommended. Small wheels may be acceptable for light trolleys that are moved only short distances on smooth floors. Width and tyre profile Narrower wheels and rounded tyre profiles roll and swivel more easily on hard surfaces. Wider treads may be necessary on soft carpets or where there are gaps that could catch a narrow wheel eg slots in drainage grates or gaps between a lift and the floor. The width of the wheel will be partly dictated by the load rating required.
Tyre material Hard materials such as cast iron and nylon have the lowest rolling resistance on hard, smooth surfaces such as concrete and are suitable in some industrial applications. However, hard wheels are more difficult to start when obstructions such as a stone or a gap in the floor is encountered. They can also be very noisy. Softer materials tend to even out the peak forces and may feel easier to push, even if the rolling resistance is higher on a smooth surface. Shock absorbing materials such as rubber or polyurethane may be required for rougher floors and outdoor surfaces. In hospitals and hotels, non-marking rubber or polyurethane may be required to reduce noise and protect floor surfaces (nylon wheels are suitable if used exclusively on carpet). Pneumatic tyres roll easily over bumps and unpaved surfaces and may be preferred for some outdoor applications. However, they have higher rolling resistance on smooth floors. Check them regularly to maintain the correct inflation pressure. Some softer tyre materials may have high friction on some floor surfaces and make it hard for the wheels to swivel into alignment when the trolley is started, resulting in a higher initial force. For high load applications it is important to try out different wheels on the floor where they will be used, before purchase. Bearings Sealed precision ball bearings provide the lowest rolling resistance. They are recommended for handpushed trolleys that are used frequently or over longer distances. Pre-lubrication and effective sealing ensure that the low rolling resistance is maintained without the need for further lubrication. There are other types of cheaper, lower grade ball bearings available but these must be regularly lubricated. Roller bearings are more commonly available for industrial castors but need periodic lubrication to maintain low rolling resistance. Plain metal bearings are acceptable on trolleys moved only infrequently over short distances, but the rolling resistance is higher than ball or roller bearings and increases markedly if not regularly lubricated. Plastic (usually nylon or acetal) plain bearings are acceptable for light loads and don’t require lubrication. Thread guards should be used to stop bearings from becoming clogged when used in environments where there are fabrics and lint eg laundries. They are also reasonably effective at keeping dust and debris out of unsealed bearings, therefore requiring less need for frequent maintenance. While taking cost and availability into account in the final selection of bearing types, you may find that the higher initial cost of sealed ball bearings is justified if push/pull forces are lower over the life of the
trolley. Contact wheel and castor manufacturers for advice on selecting the type of wheels and bearings for a particular application. Brakes Brakes on at least two wheels are important if the trolley has to be loaded/unloaded on sloping surfaces or where it is important to stop movement while transferring large items. Castors are available with total brakes that prevent swivelling of the castor as well as rotation of the wheel. ► Castor arrangements Swivelling castors allow trolleys to be steered and manoeuvred in different directions. It is important to design trolleys so that they can be manoeuvred easily with a minimum requirement for push/pull and twisting forces to keep the trolley going in the right direction. Different arrangements of swivelling and fixed wheels have advantages and disadvantages in different situations as described below. Four swivelling wheels This is the most manoeuvrable arrangement and can be moved in any direction. It is suitable for short distances in congested or confined spaces on level floors but not well-suited to long distances as it is harder to steer. On sloping surfaces it may drift sideways and require twisting effort to keep the trolley straight. Two swivel, two fixed wheels This arrangement is best suited to long distance pushing and sloping or uneven surfaces. The swivelling wheels should be at the handle (rear) end of the trolley so that you can steer by a light sideways force rather than a more hazardous twisting force. The same effect can be achieved with a four swivel trolley if one or both of the front wheels of the trolley are fitted with directional locks. Four swivel, two fixed centre wheels This arrangement is best for long trolleys. The trolley pivots in its own length and is easy to steer around corners in a passageway, but is not easily manoeuvred into a corner or parked against a wall. A variation of this arrangement, with only one wheel at each end, is acceptable for relatively narrow trolleys that are uniformly loaded. ► Handle design The optimum height for a handle for pushing and pulling is between 910 mm and 1000 mm above the ground, depending on the height of the workers. In general, the handle should be a little below elbow height. A middle height of 950 mm is a good compromise for most people. Vertical handles, instead of a horizontal bar, allow users to find their own most convenient height and should be about 450 mm apart to ensure good control of the trolley. The vertical corner posts of a
trolley frame should not be used as handles because this exposes the workers’ hands to contact with doorways and other objects. The handle should protrude at least 200 mm from the back edge of the trolley to provide room for a normal walking stride without the shins hitting the bottom edge of the trolley. Trolleys with swivel castors at both ends may also have handles at both ends to increase manoeuvrability in confined areas. ► Manual handling while loading/unloading Arrange shelves and load platforms to make it easier to transfer items to and from the trolley. Ideally, handling of loads should be done without stooping or twisting, and with hands between mid-thigh and waist height. A trolley with a platform around 800 mm high satisfies this requirement for handling heavy cartons, but a low platform, about 250 mm high is better for handling items like suitcases that have a handle at the top. Smooth shelves without a lip allow objects to be slid on and off easily. This makes it easier to handle large objects such as cartons. ► Trolley dimensions Trolley dimensions will be determined primarily by practical considerations, but the overall dimensions of the trolley should be limited so that, when full, it can be pushed without exceeding the recommended pushing forces. In addition: -
You should be able to see over the top of a trolley so that you can see where you are going. If you can’t see over the top you are more likely to adopt a twisted posture to pull the trolley or to see around it. A maximum laden height of 1300 mm will enable nearly all workers to see over the top. If the trolley must be taller than this, the sides should be open or have mesh areas so that you can see through it.
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The overall width should be at least 80 mm smaller than the narrowest doorway the trolley will pass through. However, to ensure stability, the distance between the axles of castors when both swivel castors are pointing inwards should be at least two thirds of trolley width and one sixth of trolley height, or one fifth of trolley height if trolleys are used on slopes up to 6 degrees.
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The length of a trolley should generally be between 1.5 and 2.0 times the width for ease of steering.
TROLLEY DESIGN CASE STUDY Coin handling trolley This trolley provides an example of how potential manual handling risks can be eliminated through design. A trolley was required for moving coin tins and bags of coins in a bank. Coins in bulk are very heavy so it was necessary to decide on a size that would limit the weight to a safe level for pushing.
A limit of 350 kg was chosen and it was found that this weight of coins would fit on a tray of 820 mm x 520 mm. The tray was set at a height of about 900 mm so there would be no need to stoop or twist while handling coins. The lower deck of the trolley was made to slope down from a peak in the middle so that it couldn’t be used for carrying anything. It was possible to leave the frame open at the bottom, but workers might have been tempted to place boxes on the frame and carry extra loads, exceeding the safe design limit. It is safer to prevent overloading with this design feature than to rely on workers not to use the lower deck. A six-wheel arrangement was chosen with two fixed castors in the centre and four swivel castors in the corners. This gave accurate and easy steering and made the trolley double ended – it also has handles on both ends. Most of the weight is supported on the two large (200 mm) wheels, which roll easily on carpeted floors. All wheels are nylon with roller bearings.
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