Concrete Mixer Problems
This guide covers the 15 concrete mixer faults we encounter most often in the field at concrete batching plants, with the symptoms, possible causes and checks and fixes for each. Select the heading closest to your problem from the side menu.
Safety warning: The checks and interventions in this guide must only be carried out by trained maintenance personnel. Before entering the mixer or working on the hydraulic, pneumatic or electrical system, stop the plant, isolate the power and apply the LOTO (lockout/tagout) procedure; release the pressure in the hydraulic and pneumatic lines. If in doubt, contact MEKA technical service.
Mixer motors trip the thermal overload on start-up
Symptom: The mixer main motors draw high current at start-up and trip the thermal overload.
Check inside the mixer for aggregate or cement leakage. Adjust the clearance between the paddles and the liner plates to 3–5 mm. Also check the current setting of the motor protection circuit breaker.
Rhythmic metallic knocking from the mixer body
Symptom: Rhythmic metallic knocking comes from the mixer body during operation.
Check the mixing arm fixing bolts for loss of torque and the paddles for breakage. Take the necessary safety precautions, apply the LOTO (lockout/tagout) procedure and enter the mixer. Tighten the high-strength fixing bolts with a torque wrench to the torque value specified by the manufacturer.
Mortar leaking from the main shaft seals
Symptom: Mortar/slurry leaks out of the mixer main shaft seals.
Check the grease pressure at the shaft bearings. Check whether the seal (packing) set has reached the end of its service life or has been damaged by back pressure. If necessary, replace the complete seal kit.
Hairline cracks at the body bolts and chassis joints
Symptom: Hairline cracks are visible at the outer body bolts and the chassis joints.
Check for signs of high vibration and metal fatigue. Grind and prepare the cracked area properly before welding, then repair it with gas-shielded (MIG/MAG) welding of a quality suitable for X-ray inspection. After the repair, check the mixer mounts for loosening, deformation or damage.
Holes worn through the liner plates
Symptom: There are local holes in the mixer's inner side and floor wear liners (plates).
The liner plates have reached the end of their service life (wear limit). Before replacing them, confirm that the casting quality of the new plates (Ni-Hard 4 or chromium alloy) meets the technical specification. Remove the worn plates without damaging the body shell and replace them with a complete new set.
Mixing paddles wear quickly or break
Symptom: The mixing paddles (blades) wear and break much faster than normal.
This is caused by a high quartz (silica) content in the aggregate or an unsuitable paddle angle. Optimise the paddle angle according to the recipe values and increase the casting hardness to improve wear resistance.
Washout nozzles spray at low pressure
Symptom: The mixer's internal washout nozzles (high-pressure washing system) spray water at low pressure.
The water nozzles may be clogged with limescale or mortar dust, or the high-pressure pump may have drawn in air. Remove the nozzles, clean them in an acid bath and check the pump.
Mixing arm bolts shear
Symptom: The bolts fixing the mixing arms to the mixer body shear during operation.
Checks and fixes:
- Check the mixing arm bolts and connection points
- Identify mechanical stress caused by loosening, wear or misalignment
- Use new bolts of the correct strength class
- Tighten the bolts to the manufacturer's torque values
- Check the alignment of the mixing arms and connecting parts
- After maintenance, test-run the mixer empty and under load
Heavy dust escaping from the top cover seals
Symptom: During batching, heavy dust escapes from the mixer top cover seals.
The top cover sealing sponges/EPDM seals are deformed. Check that the filter extraction fan is working and service or repair it if it is faulty, then replace the EPDM seals on the cover with new ones.
Discharge chute liner worn through and leaking mortar
Symptom: The inner liner of the mortar discharge chute has worn through and mortar leaks underneath.
Checks and fixes:
- Check the wear level of the chute's inner liner
- Replace the worn-through, unusable liner with a new one
- Check the areas of the chute's inner surface that are causing the wear
- If necessary, use a new liner material with higher wear resistance
- After fitting, check the mortar flow and sealing
Mortar forms lumps during mixing (balling)
Symptom: Lumps (balling) form in the mortar inside the mixer during mixing.
Checks and fixes:
- Check the mix proportions (water, cement, aggregate and admixture quantities)
- Inspect the wear of the mixer paddles and mixing elements
- Set the mixing time and mixer speed to suitable values
- Check the material charging sequence
- Clean out hardened mortar build-up inside the mixer
- After adjustments and maintenance, test the homogeneity of the mix
Mortar build-up does not come off when washed
Symptom: Mortar layers left inside the mixer do not come off even after washing with water.
The washing water pressure (bar) is too low or the chemical retarder dosage is insufficient. To fix the problem:
- Connect an additional booster pump to the washing system
- Readjust the chemical dosing times to suit the system requirements
- Check the washing performance after the adjustments
Drive belts squeal and overheat on start-up
Symptom: The mixer drive motor belts squeal at start-up and overheat.
The V-belt tension has dropped or the pulley grooves are worn. To fix the problem:
- Check the V-belt tension with a suitable belt tension gauge
- Set the belts to the ideal tension specified by the manufacturer
- Replace worn or deformed pulleys
- After maintenance, check the belt–pulley alignment and running performance
High-frequency grinding noise from the motor bearings
Symptom: A continuous high-frequency grinding noise comes from the mixer motor bearing housings.
The bearing may be short of grease or have damaged balls. Take measurements with a clamp meter and a vibration meter, and replace faulty bearings with SKF/FAG or equivalent quality bearings. Checks and maintenance steps:
- Check the motor bearings for vibration and noise
- Inspect the bearings for wear, play, damage or insufficient lubrication
- Check the lubrication and grease level of the bearing housings
- Replace damaged or worn bearings
- Check the motor shaft alignment and the bearing arrangement
- After maintenance, monitor the motor's running noise, vibration level and temperature
Current imbalance between the motors of a twin-shaft mixer
Symptom: On twin-motor (twin-shaft) mixers, there is an unbalanced current (amp) difference between the motors.
Checks and fixes:
- Check and compare the running current of both motors
- Check the motor supply voltage, phase balance and electrical connections
- Inspect the gearboxes, couplings and shaft connections for mechanical strain or binding
- Check the mixer shafts, paddles and bearing arrangements
- Remove any mechanical resistance that could overload one of the motors
- If necessary, check and adjust the motor protection and drive parameters
Still not solved? If the fault persists after these checks, you can reach the MEKA technical service team 24/7. For the genuine parts you need, see Concrete Batching Plant Spare Parts.
Other concrete batching plant troubleshooting guides:
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