Our ergonomists conducted a hazard assessment for a road crew, part of which included a worksite which involved removing manhole covers. The task was a one-person job that involved using a lever to remove manhole lids.
The lever itself is approximately 54” (137cm) long with an adjustable 50” (127cm) chain; the entire mechanism weighs about 14.5 lbs. A hook at the end of the chain attaches to the manhole lid. Adjusting the chain to a shorter length increases the lifting motion to pull the manhole lid upwards. Using a longer chain length allows the worker to pull and drag the lid away from the hole once it has cleared the base.
To remove manhole covers, most workers used their own body as a counter weight. They used a two-handed grasp on the lever handle, extended one leg behind them, and leaned backward, pulling the lever to lift the lid. When the lid opened from the base, the worker would then pull the lever toward the body to lift it completely off the hole. Often the worker had to repeat this motion 2-3 times before completely removing the cover.
Hazard Assessment: Removing Manhole Covers
Adding to the difficulty of the task was:
- Weight of the manhole lid – steel lids are heavier than aluminum
- Design of the lid – impacts the use of leverage since it must be lifted straight up first before dragging it horizontally away from the hole.
- The current tool being used – it is difficult to lift and drag simultaneously. A shorter chain will be able to lift the lid up but then it makes it harder to drag it away unless it is lengthened.
- Frozen mud, dirt or ice sealing the lid to the base.
The ergonomist used a force gauge to determine the amount of force required for the initial pull to remove the manhole cover. The requirements actually exceeded the gauge’s capacity of 51 kg or 112 lbs, which exceeds most standard guidelines.
Our team came up with the following recommendations:
- Use a blowtorch to melt frozen debris and ice around the manhole lid, taking care not to overheat and expand the lid. Using a sledgehammer to remove debris is an alternative.
- Improve the tool’s leverage – attach a small pipe at the angle of the lever and attach the chain to this pipe instead. The lid lifts vertically and then the cover is dragged away from the hole by following through with a pull.
- Another option is a type of lift that uses a magnetic or chain device that mechanically lifts the lids from the manhole cover. We usually recommend trying out a few different designs with the workers first. Therefore, consider the following design principles:
- The device should be portable, lightweight, and easy to set up.
- The device should let the user stay in an upright stance as they apply counter weight to the lever during both the lifting and dragging phase.
- The hook or end of the device, which attaches to the lid, should remain secure to the lid as the device is in use.
- Leverage should ensure that the direction of force against the lid is first vertical, followed by horizontal, or angled away from the direction of the hole.
- Remember to trial the device with the workers first and measure the force to make sure it is still within standard guidelines.
- Other points to remember to tell your workers:
- Examine condition of manhole before lifting.
- Continue to use body weight to counter weight the device rather than pulling only with the shoulders and arms.
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