What are flexures and when can they be used?

Vessel and hopper installations normally require connections to supply and discharge product. These pipes, conduit or conveyances are called "flexures" and can be stiff, which can cause non repeating binding errors on your weigh system. They affect the amount of weight placed on the load cells and in fact cause a weight division as some weight will be directed to the pipe or attachment and some weight directed to the load cell. The amount of weight on the pipe or attachment is not weighed and the scale system weight readings are incorrect. In vessel design it is imperative to minimize these types of mechanical binds or rubs that divide the weight force away from the load cells. There are sections of supply and discharge piping that are designed to allow small amounts of movement and commonly called flexure's.

The general rule is if you can shake any attachment to a weigh vessel with minimal effort it should be OK.  It can become a problem if you have too many of these semi-flexible connections.  The accumulated connection resistance results in a significant weighing error.

1. Look for piping or tubing that may hang over a sharp edge and catch as the vessel raise and lowers with weight exchanges.

2. Make sure that the flexures are not pressurized and act as a stiff pipe in one instance and flexible the next instance. Hydraulic and water lines may be made of rubber, but when placed under pressure cause a weight division error that is difficult to find.

3. Monitor the vessels under operating conditions and verify that there are no paths for weight to travel away from the load cells.

4.  Pipes and conduit connections should not push or pull on the scale.  When installing pipes and conduit to scales, start connecting at the scale end and work out.  Insure that connection is not a weight divider.

There are is a second type flexures in process weighing, including:

1. A system using levers and beams, and a single load cell to give a ratio of the weight and calculates the total load and must use hard calibration.

2.  A system with multiple legs and using fewer load cells than legs and must use hard calibration.

Flexures are used in weighing applications where level indications are the main reason to have the vessel weighed. Levered systems would not be desirable for batching applications where speed and accuracy are critical. Levered systems require frequent weight verification test due to mechanical wear of the knife edges and pivot points.  Using fewer load cells does increase the stability of a vessel and reduces the cost of additional load cells. The cost difference is small because some other type of mechanical points and support will be required. Pre-manufactured mounts normally cost approximately 60% that of the actual load cell and mount.
Flexures are semi-rigid support points which may be used to replace some of the load cells on a vessel to improve stability or reduce cost of the installation, when the following conditions are met:

1. The material to be weighed is a liquid or product which is self leveling. If the weight was not self leveling the product could be centered over the flexured portion of the vessel that is not weighed.

2. The weighing vessel is symmetrical and without partitions.

3. The center of gravity is ½ way between the flexures and the load cells.

4. The center of gravity does not move horizontally.

5. The vessel is free standing, no weight is supported by piping or other load paths.

6. C2® calibration will not work on a Flexured system. C2® load cells must see the total weight. C2® Load cells and flexures share the weight of the hopper.

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