Mortar mixer: the paddle does not know your hand

A mortar mixer's paddles turn about 30 times a minute with enough torque to fold a shovel in half. They do not slow down for a glove.

The points

• Hands and arms go into mortar mixers when someone reaches in to clear a clump or knock a bag off the paddle with the engine running. The paddle catches the glove and takes the arm around the drum.
• The grate over the drum stays on. It is the guard that keeps a shovel, a bag and an arm out of the paddles, and a mixer with the grate off or wired open does not run.
• Nothing goes in the drum but sand, cement, lime and water while it turns. Nothing comes out of it, no shovel, no hoe, no hand, until the engine is off and the paddles have stopped.
• Cleaning the drum is engine off, key out or plug pulled, and the paddles stopped. Chipping mortar out with the engine idling and the clutch out is one slip away from an amputation.
• The skid steer or loader bucket never swings over the mixer while someone is at it. The operator and the mixer tender can see each other, and the tender steps back and signals before the bucket comes in.

Why it goes wrong

The paddles in a 6 to 9 cubic foot mixer are driven through a belt or hydraulics by an engine that does not care what it is turning. The mixing force that folds stiff mortar folds an arm the same way. Most mixer amputations are a hand reaching in to free a clump, a bag or a shovel while the drum is turning, and the paddle takes the glove first.

29 CFR 1926.300 says belts, gears, shafts, pulleys, sprockets, spindles, drums and other rotating or moving parts are guarded if they are exposed to contact. On a mixer that is the grate over the drum, the belt guard over the drive, and the shaft guard. A grate that is hinged so the mixer stops when it lifts is the best kind. A grate that is missing means the mixer is down.

29 CFR 1926.702 adds that no maintenance or repair happens on concrete and masonry equipment until the hazardous energy is locked out and tagged. Cleaning is maintenance. Key out and in a pocket, or the plug pulled and in sight, before anyone puts a hand or a tool in the drum. A gas mixer that is off but not locked can be started by the next person who walks up.

The loader bucket over the mixer carries 1,000 to 1,500 pounds of sand over the tender's head. The pinch between the bucket and the drum is a crush, and the operator's view of the tender is blocked by the bucket at exactly the moment it matters. The tender steps back and signals, the bucket comes in, the bucket goes out, the tender steps in. Sand does not get dumped with a person at the drum.

The dump handle and the drum are a pinch point for the hand that is steadying the tub while the other hand tips the drum. The mixer sits level on firm ground, wheels chocked, not on the edge of a slab or a trench where it can walk while running. And the mortar itself is wet cement, so gloves and the same washing as the concrete crew.

End of day is when the shortcuts happen. The drum is chipped clean with the engine running because it is faster. The shovel is left in to knock the mortar loose. Both of those are the injury this talk is about, and both are avoided by shutting the engine off first and waiting the ten seconds for the paddles to stop.

Questions for the crew

1. Is the grate on our mixer today, and does the mixer stop when the grate lifts or is it just a grate?
2. Who is feeding the mixer with the loader, and how does the tender tell them to hold?
3. When the drum gets cleaned at the end of the day, who shuts it off and how?

To close

Nothing goes into the mixer drum today except through the grate, and the engine is off before anyone's hand or tool goes near the paddles.