Rebar Tying Machines - Part 2

In Ontario, rodworkers have a higher proportion of lost-time musculoskeletal injuries to the back and upper limbs than all other construction trades combined (Figure 1). The total cost of their lost-time injuries and the amount of their time off work are also greater than for all other construction trades combined.

Figure 1: Non-traumatic musculoskeletal injury by construction rate group (WSIB data: 1994-1998).

CSAO research has shown how to improve this situation: reduce the frequency of awkward trunk postures, particularly during ground-level rebar installation.

One way to improve posture is to use an automatic rebar-tying machine or "gun." This is an electric tool (cord or battery) that can be applied where bars cross. When you press the trigger, the machine feeds the wire around the bars, twists it, and cuts it (Figure 2).

Figure 2: Notice the difference: working posture while
manually tying rebar (left) and while working with the
rebar-tying machine (right).

The WSIB Research Advisory Council (RAC) has allocated research funds to study

• the effectiveness of the rebar-tying gun in reducing risk of injuries
• the potential use of the gun as a rehabilitation tool for injured workers.

The research project is divided into three phases. The first phase focused on a controlled laboratory experiment investigating the biomechanical differences between manual tying and using the rebar-tying machine. In the second phase, field experiments will investigate the long-term health benefits of using the gun. The final phase will field-test the machine as a rehabilitation device to assist injured workers in their return to work. This article presents the results of the first phase of the study.

Phase 1 Study -
Controlled Experiment
To evaluate the risk of low-back injury and the benefits of using the rebar-tying machine, CSAO conducted a controlled experiment with nine apprentices. The experiment took place at the International Association of Bridge, Structural, Ornamental, and Reinforcing Ironworkers Local 721 Training Centre in Toronto.

To evaluate rebar-tying tasks and determine the potential for reducing the risk of injury, the research team employed electromyography to measure low-back muscle activity. This activity was used to quantify low-back compression at the L4/L5 disc joint.

Results
Working with the rebar-tying gun significantly decreased peak loading in the lower back at the L4/L5 disc joint (Figure 3). Cumulative loading on the back was also significantly less than during manual tying with pliers (Figure 4). These reductions in low-back loading were mainly due to the workers' upright position while using the gun with an extension (Figure 2). Numerous studies have shown that reducing peak and cumulative forces on the lower back can significantly decrease the risk of low-back injury.

Figure 3: Using the rebar-tying machine resulted in significantly less peak loading on the lower back (L4/L5 disc).

Figure 4: Using the rebar-tying machine resulted in significantly less cumulative loading on the lower back (L4/L5 disc).

Conclusion
Rodworkers have a high rate of lost-time claims due to musculoskeletal injury. The high rate may be related to the frequency and repetition of two factors:

1. the manual handling of heavy material, and
2. the static, awkward posture connected with ground-level rebar tying.

To lower the risk of musculoskeletal injuries, control measures must be designed to reduce physical exertion, such as frequent heavy rebar handling and awkward trunk postures, particularly during ground-level work.

Phase 1 of this study indicates that one way to improve the situation is to use automatic rebar-tying machines. Results indicate that working with the machine can decrease repetitive wrist activities such as the bending, twisting, and lateral bending associated with manual tying. Furthermore, using the gun with an extension can decrease static bending of the trunk, which in turn decreases peak and cumulative compression forces on the lower back.

The research team concluded that using the automatic rebar-tying machine can significantly decrease the risk of musculoskeletal injuries to rodworkers.

Limitations have become apparent, however, in real job situations.

1. The MAX RB392 can only tie two rebar having a combined area of 150 mm sq. to 300 mm sq. This allows tying of only about 40%-60% of the bar used on a typical project.
2. The gun requires regular maintenance. After two to three days of use, the tie-wire that spools out and along the jaw of the gun constantly jams on its inner mechanics. Once jammed, it is difficult to clear the wire without the assistance of the manufacturer.

MAX Tools (www.maxusacorp.com) has developed a bigger rebar-tying gun (RB650) that can tie a combined area of 58 mm sq. to 750 mm sq. Preliminary tests show that many of the problems with the smaller gun have been solved. CSAO will conduct additional research in real job situations during Phases 2 and 3 of the study.