Why Shimming?
Peristaltic pumps utilize rollers or shoes to compress and release a hose in a controlled manner, creating a vacuum that draws fluid through the hose. Shimming, the process of adding or adjusting thin strips of material (shims) for precise pressure control to close the hose, is critical in peristaltic hose pumps, especially those utilizing rubber hoses.
Uniform compression might not be achieved without proper shimming in peristaltic pumps. Even with high manufacturing standards, there can be slight variations in the dimensions and geometry of pump components, including the rollers, shoes, and the pump housing. These variations can lead to uneven compression of the hose if not corrected by shimming.
Rubber hoses themselves can have variations in thickness, flexibility, and resilience, even if they come from the same manufacturer. These variations can affect how the hose compresses under the rollers or shoes. Shimming helps in adjusting for these hose variations to ensure consistent compression.
The components of the pump, including the housing, rollers, shoes, and the hose, can expand or contract due to temperature changes during operation. This thermal expansion can affect the gap between the hose and the rollers or shoes, potentially leading to uneven compression if not accounted for with proper shimming.
Shimming adjustments are typically made during the pump setup or maintenance to ensure that the pump can operate efficiently across its entire performance curve. When the pump is running at a specific RPM, the shims are selected and adjusted to ensure that the shoes compress the hose properly, even as the discharge pressure changes. The correct shimming is vital to ensure that the hose is fully compressed throughout each revolution, creating a peristaltic action to move the product without backflow or slippage, which would indeed affect both performance and hose life.
Installing a hose correctly also involves correct shimming. Follow the following steps for a more correct and easy shimming.
- Tolerances (Hose, Pump House, Rotor, Pressing Shoe/Roller) – Measure A Dimension in 3 locations and take the largest number!
- Hose Finish: Wrapped vs. Rectified Hose (There is a difference in the number of shims to be needed)
- Peristaltic Hose Tolerances (Always check the ID of the replacement hose!)
- Hose Material Expansion with Warm Product Transfer (reduction of 1 shim)
- The speed of the pump also changes the pressure – always check required shims on the differend speeds and pressure
- The ΔP = Discharge Pressure − Suction Pressure of the pump – ensure you have both inlet and discharge pressures at Peak Pulsation Pressures
- High viscosity causes more resistance in small cavities, makes possible backflow more difficult (reduction of 1 shim)
- Shims calulated are for 1 shoe/roller each only – multiply the total number of shims required by the number of shoes/rollers of the pump.