How to Specify and Verify the Inside Circumference of a Soft Silicone Belt

A soft endless silicone belt needs more than a nominal length on its drawing. For an inside-circumference requirement to be useful, the buyer and manufacturer must also agree on the measurement state, applied force, reference surface and acceptance limits. The following checklist helps turn a dimensional request into a repeatable pre-shipment inspection.

Define exactly which length is required

Describe the dimension as the complete circumference along the belt’s inner surface, and identify the measuring track across its width. Keep this separate from outside circumference, a reinforcement or pitch-line length, and the overall end-to-end dimension of a flattened loop.

Measuring the inner surface is an established flat-belt convention: the Forbo flat-belt technical guide describes placing its endless spliced belts on edge and measuring around the inside. That guidance illustrates the reference surface; it does not establish a complete acceptance procedure for every soft silicone construction.

Do not automatically double a laid-flat end-to-end measurement. The folded ends, belt thickness and chosen reference surface make that shortcut unsuitable as proof of inside circumference unless the specific method has been validated.

Agree the measurement state and force

Choose either a defined free-state measurement or a measurement under a specified load. Record the choice on the drawing or inspection plan. Free-state measurement means no intentional tensile stretching, with a documented support and handling method.

For a loaded measurement, approve a numerical force in newtons, its permitted variation and where it is applied before inspection. State whether it is the force separating the fixture rollers or the tension in one belt span. These quantities must not be treated as interchangeable. “Pull lightly” and “just taut” are not reproducible load specifications. Until these settings are approved, treat a loaded-length reading as preliminary rather than an acceptance result.

Also agree on:

• Conditioning temperature, duration and relevant environmental conditions
• Time since manufacture and recovery after packaging or previous stretching
• Loading sequence, any seating cycles and the delay before reading
• Number of repeat measurements and how their variation will be reported

ISO 23529 provides general preparation, measurement and conditioning guidance for rubber test pieces, including timing between forming and testing. Its scope excludes special requirements for whole-product testing, so the belt-specific procedure still needs agreement.

Make the circumference method repeatable

For a direct inner-path measurement, document how the loop is supported and how the tape follows the entire inner surface. Check that the measuring tool does not visibly stretch, indent or distort the belt. Repeat the setup rather than taking several readings without disturbing it.

If a roller fixture is selected, document roller diameters, alignment, contact geometry, force application and the calculation that converts the measured spacing into inner circumference. Validate the calculation for that fixture and reference surface. Record the actual force and reading time with the result.

Keep installation tension separate from dimensional inspection. An installed, stretched belt cannot be compared directly with a free-state acceptance limit. Do not borrow measuring loads or length conventions from V-belt or timing-belt procedures without establishing their applicability to the custom flat belt.

Specify width, thickness and acceptance limits

Give width and thickness their own limits and measuring locations. For contact measurements, specify the instrument, contact geometry and force so that operators do not squeeze the soft material differently. Record individual readings where local variation matters.

State the minimum and maximum acceptable inside circumference, the reporting resolution and the agreed decision rule for results near a limit. Manufacturing tolerance and measurement uncertainty need separate consideration.

ISO 3302-1 covers dimensional tolerance classes for molded, extruded and calendered solid rubber products. Any reference to it should identify the applicable product process and agreed class; it does not replace a clear belt drawing.

Check whether the hardness test is suitable

A hardness reading needs a named scale, method, specimen identification, support arrangement and reading time. ASTM D2240 treats durometer hardness as an indentation measurement influenced by the indenter, applied force and material response. It does not provide a universal conversion between hardness scales or a direct measure of belt tensile stiffness.

A thin or curved belt may need a different specimen arrangement or test method. ISO 48-4 distinguishes durometer scales, including AM for thin rubber test pieces in the normal-hardness range. Have the laboratory confirm suitability rather than substituting a scale silently. If an agreed companion coupon is tested, identify it clearly and document its relationship to the belt material and cure.

Ask for photographs that support the report

Request an overview of the complete setup, close-ups of the length reference and readable scale, width and thickness measurement positions, and the hardness instrument with its specimen support. Include the specimen identifier and inspection record reference.

Photographs supplement recorded readings; they cannot establish calibration, conditioning or force history by themselves. When discussing silicone conveyor belt requirements with IKSONIC, include the agreed inspection method alongside the drawing so both parties can evaluate the same dimension under the same conditions.