And this is the charge of their burden, according to all their service in the tabernacle of the congregation; the boards of the tabernacle, and the bars thereof, and the pillars thereof, and sockets thereof, (Numbers 4:31)
The duty of the Merarites was the transport of the actual framework of the tabernacle itself. They too used wagonsons (ch. 7:8). Their burdens would be much heavier than those the Kohathites carried, for they were all the solid parts of the structures, with their accessories.