6 Common Questions New Users Have About Volumetric Mixers
1. What happens when you hear a pop and the concrete becomes loose and has no cement?
Some units have a shear pin that protects the drive system. The popping sound indicates the pin has broken and needs to be replaced. Several factors could cause the shear pin the break, including:
- The cement was not fluffed and packed around the augers;
- Frequent starting and stopping of the system; and/ or
- Material in the bin, such as a hard piece of cement or piece of cement bag.
Routine maintenance is necessary on all machines, and replacing the shear pin is an item to account for.
2. What is wrong if the conveyor will not run and seems to be stuck?
Check to see if cement buildup is plugging the cement transfer auger. The auger runs with the same hydraulic oil used for the conveyor; if one motor doesn’t run, the other won’t run. Clean the discharge spout to see if that fixes the problem. Another cause can be if the conveyor system has become too loose and the chain catches on the front chain rail. Run the conveyor backwards to release the chain from the rail and then adjust the conveyor tensioners. Make sure to tighten both sides equally.
3. How do you set the gates for a mix?
Gates are adjusted to the proper indicator marks based on previous calibration. Reference the O&M manual for calibration instructions. With proper gate settings per the mix design requirements, correct and accurate concrete and flowable fill will be produced. You can be assured of accuracy and replication.
4. Why is the conveyor spilling sand or stone under the truck?
When sand or stone spills, adjust or replace the material side guides. These guides prevent spillage and keep the aggregates on the belt system. When adjusting the guides, be careful not to make them too tight; there must be an air gap between the guide material and the belting. Units that have the guide material setting at a 45-degree angle have no adjustment to the guide rubber. You will need to replace the guide material.
Along with the material side guide, units have a belt wiper at the discharge end of the swivel hopper. This wiper must be tight against the conveyor so that it can wipe the belt clean and not carry materials under the truck. Front wipers lie on top of the belt after it has rounded the front idler sprockets. If this wiper is damaged, material will spill off the belt when driven down the road.
5. When should I replace the wear blades on the mix auger?
Make sure the blade does not wear pass the bolt hole that holds the blade to the flighting and mounts. If the blades wear to within 1/8-inch of the countersunk hole on the wear blade face, then they need to be replaced. Be sure to watch the first half of the auger, as it will have the most wear. The discharge blades may last twice as long as the first half. Once the material mixes together, it’s less abrasive and wears less on the blades.
When replacing the blades, over tightening the bolts can cause them to crack. Make sure you use the spacer washers between the blade and flighting.
6. What mix angle should the mixer run?
The mix angle directly affects the quality of the concrete. Mixers need to run at a good mix angle for retention, with an ideal angle of 25 degrees to 30 degrees and no less than 15 degrees. The higher the angle, the longer the retention of the concrete and the better the mix. If you could see the concrete, it would be rolling back and moving in a Figure 8 pattern through the homogenizing auger mixer. If the mix continually backs up in the mixing auger, lower the angle. Longer retention in the mixer will help:
- Admixtures have time to react,
- Stop any false/flash in the concrete, and
- Reducing bleed water.