Tag: wheels

How Force Affects Pushing and Pulling Activities

Pushing and pulling tasks are among the most common industrial activities. Pallets of goods need to be moved from one point to another and equipment needs to be moved to a usage point. Workers at factories, hospitals, distribution centers, grocery stores and many other businesses engage in pushing and pulling activities numerous times a day. The Ergonomics of Manual Material Handling – Pushing and Pulling Tasks provides a useful overview of the costs and consequences of neglecting ergonomics in common industrial tasks that involve pushing and pulling. Click here to read the white paper published by Darcor, an industry leader in the design and manufacture of ergonomic casters and wheels, and Ergoweb, an ergonomic web resource.

While often taken for granted, wheeled carts and equipment are integral to the operation of nearly all manufacturing and distribution facilities as well as many businesses. Musculoskeletal disorders from pushing and pulling injuries cost American businesses billions of dollars each day in medical, insurance, disability and downtime costs. Ergonomically-designed carts, wheels and casters can significantly decrease the incidence of musculoskeletal disorders.

To be effective, ergonomic design for push/pull tasks must consider:

  • Human factors such as height, weight, age, gender, strength, posture and physiological capacity.
  • Task factors including distance moved, forces required to initiate and sustain movement, direction and nature of movement and task duration.
  • Cart/equipment factors such as size, weight, stability, caster/wheel specification and handhold type, height and orientation.
  • Floor/ground factors including surface characteristics, slope and contaminants.

Contrary to popular belief, horizontal push force is more significant than load weight in pushing and pulling tasks. Proper wheel or caster selection and equipment design can enable workers to move thousands of pounds safely and efficiently. Caster/wheel choice alone can reduce push force significantly. Rolling resistance refers to forces that resist movement and defines the amount of force a person must generate and apply to move wheeled equipment.

This force — called the starting or initial force by ergonomists — is always greatest at the start, just before movement begins. Fortunately, starting force must only be exerted briefly. Once acceleration is achieved, less force — called the sustained or rolling force — is required to maintain movement. The final major force that affects cart movement is turning force which can occur while the cart is in motion or during positioning.

Next time: How ergonomics mitigates force.

How Ergonomics Mitigates Force

Horizontal push force is a more significant factor than load weight in pushing and pulling tasks. In moving a wheeled piece of equipment or a load on a wheeled cart, three main forces come into play:

  • Starting or initial force is the effort required to initiate movement. It requires the greatest effort because it must overcome inertia.
  • Sustained or rolling force is the effort required to keep the load in motion and requires significantly less effort if a steady speed is maintained.
  • Turning force is the effort required to turn the load and can be significant as the load is moved into a new direction, often requiring asymmetric body postures and muscle exertion which carry a greater risk of injury.
  • Stopping or positioning force is the effort required to bring the load to a halt or position it in a specific place. Positioning can require significant, multidirectional force, exposing the worker to hazardous postures and muscle exertions.

Ergonomics mitigates these forces through design that seeks to minimize stress on the worker’s body and reduce wasted movement and effort. By reducing unnecessary movements and awkward postures, the force exerted by a worker is more efficiently utilized, thus reducing the amount of force necessary to move a load. This reduces both the time and effort needed to complete a task and the risk of worker injury. To ensure that equipment can be easily used by all members of the workforce, ergonomic design is often adjustable to fit a greater number of workers or is targeted to accommodate the weakest members of the workforce.

Ergonomics works to reduce inertial and dynamic forces, friction and physical interference to decrease the amount of force required to move a load. Considerations in ergonomic design generally include:

  • Floor materials, pitch and slope
  • Load weight, type and quantity per shift
  • Cart or equipment size, weight and design
  • Wheel or caster type, construction, materials, number and placement
  • Handhold type, height, width and placement
  • Control type and placement
  • Route, number of turns, obstacles and amount and type of maneuvering
  • Frequency, duration and repetition of task
  • Body postures required to operate equipment and perform tasks
  • Height, weight, strength and gender of typical worker