How to Measure Custom Sewn Bellows: Compressed Length, Extended Length and Mounting Dimensions

To measure custom sewn bellows, record the shortest and longest distances between the same two mounting reference points, the space required around the protected component, the maximum outside envelope and the dimensions of both end fittings. For a straight axial installation, the difference between the two installed lengths is the required bellows travel.

A cylinder stroke or a photograph of an old cover is a starting point, not a complete specification. A useful drawing shows where the bellows attaches, how much space remains when the mechanism retracts and what the folds must clear throughout movement. These measurements apply across the BSTFLEX custom sewn bellows range, with additional details for round, rectangular, zippered and wire-supported constructions.

Start with the Mounting Points, Not the Loose Bellows

Identify the fixed and moving attachment points on the machine. Mark them as End A and End B on a sketch, and show the exact reference surfaces used for length measurements. A mounting face, a collar shoulder and the edge of a fabric fold are different references; do not interchange them.

Keep the same references at both ends of travel. Also state whether a dimension includes collars, flanges or mounting plates, or covers only the flexible pleated body. Use millimeters or inches consistently, and identify the controlling unit on drawings that show both.

Have authorized personnel position and secure the equipment according to its operating and maintenance procedures. Before taking hands-on measurements or removing a cover, isolate hazardous energy, verify the safe condition and secure components against unintended movement. Do not reach into an operating mechanism.

Compressed Length, Extended Length and Stroke

For this guide, Lc and Le describe the required installed lengths between the agreed mounting references. They do not describe the unverified limits of a loose cover.

The two installed end positions must both be checked. A cover may have enough extension but too much compressed bulk for the available space. Conversely, a compact folded cover may not provide the required reach. Do not assume that a relaxed bench length or the number of folds resolves either question.

The bellows should not become the machine's travel stop. Report any additional movement that the cover is expected to accommodate, such as a maintenance position or defined overtravel, and have the required allowances confirmed during design review.

A Worked Example: Keeping End Fittings Out of the Stroke Calculation

Consider a straight axial installation with a shortest mounting distance of 120 mm and a longest mounting distance of 620 mm. In this illustrative layout, a fixed end section occupies 20 mm between each mounting reference and the adjacent active fold.

The travel remains 500 mm because the same fixed offsets are removed from both measurements. Subtracting them from only one length would give the wrong result. The offsets in this example are the portions shown between the mounting references and active folds; they are not automatically equal to the full physical length of an overlapping collar.

These values illustrate the measurement method only. They are not standard BSTFLEX dimensions, an approved compression ratio or proof that a particular construction can fit. The fabric, folds, seams, supports and end fittings still need to be evaluated together. For offset, angular or multi-axis movement, provide the movement path rather than relying on this axial calculation alone.

Round Bellows: Separate Body Clearance from Collar Fit

For sewn round bellows for cylinder rods and screws, record the largest component envelope that the flexible body must surround. Include any shoulders, nuts, couplings or other features located inside the protected space, rather than measuring only the rod or screw shaft.

Then record the maximum outside diameter allowed by surrounding brackets, sensors, hoses and guards. Label the component dimensions separately from the proposed bellows dimensions so the manufacturer can evaluate running clearance, fold depth and any internal guides.

The body opening and the mounting collar opening serve different purposes. The body needs clearance around the moving assembly; the collar must fit its attachment surface. Measure each receiving surface separately, including its usable engagement length and any step or groove that affects clamping.

State whether the enclosed component rotates. A cover around a rotating screw or shaft needs a suitable clearance and guidance arrangement; it should not be assumed to rotate with the component.

Rectangular Bellows: Measure the Full Cross-Section

For sewn rectangular bellows for machine ways and linear rails, record the required internal width and height, together with the maximum available external width and height. A single width measurement does not define a rectangular cover.

Show rail profiles, bearing blocks, protruding bolts, lubrication fittings and other features within the protected area. Include an end view as well as a side view. Mark whether the cover surrounds the whole section or protects selected sides, and identify any opening that must remain accessible.

Measure both end frames independently. Do not assume that their openings, outside dimensions or mounting-hole patterns are identical. Note corner radii, offsets and any obstruction that limits access to the fasteners.

Record the Mounting Dimensions at Both Ends

Label the following details End A and End B, even when the fittings appear symmetrical. State the viewing direction for each end sketch so that an asymmetric hole pattern or zipper position cannot be interpreted in reverse.

Clarify whether mounting hardware will be reused or supplied with the new cover. A close-up photograph helps identify the arrangement, but the hole pattern and mating dimensions still need a dimensioned drawing or verified measurements.

Zippered Bellows: Include the Opening and Access Requirements

When specifying sewn zippered bellows for retrofit installation, add the longitudinal opening position to the end view. For a round cover, a clock position is useful only when the viewing direction is also stated.

Identify the space available to operate the slider, open the cover and reach the end fasteners. Note nearby guards, coolant outlets and hot surfaces that may influence the closure position. Measure the working profile with the zipper closed; the width of an opened fabric cover is not its installed inside diameter.

Check the end fittings as well as the fabric opening. A longitudinal zipper can simplify fitting around an existing component, but a one-piece mounting frame may still require disassembly. Installation access must be evaluated for the complete assembly.

Wire-Supported Bellows: Add Orientation and Support Information

For sewn wire-supported bellows for large-diameter applications, provide the same inside, outside, compressed and extended dimensions, then identify whether the installation is horizontal, vertical or inclined.

For replacement work, record the existing support arrangement and spacing where accessible, and state whether that spacing was measured with the cover extended or compressed. Include photographs of damaged supports, rubbing marks or sections that sagged.

Do not use wire reinforcement as a reason to omit guidance checks. Ask whether the planned span and movement need separate guides, suspension or intermediate support. Include the space available for these components rather than treating the fabric diameter as the complete installation envelope.

Add Operating Conditions to the Dimension Sheet

Provide movement speed, acceleration where known, cycles per minute and installation orientation. Identify normal ambient conditions, exposure near hot equipment, coolant or oil contact, abrasive dust and chips. For heat exposure, distinguish surrounding air temperature, radiant heat and any possible surface contact.

The selected fabric, coating, sewing thread, zipper and support components must suit the application together. A temperature value for the fabric alone should not be treated as the working limit of the assembled bellows.

For a closely enclosed or fast-moving installation, identify how air can enter and leave as the cover changes volume. Request a ventilation review rather than assuming air will pass adequately through the seams or zipper.

Using an Existing Bellows as a Replacement Reference

Keep the old cover as a reference for profile, end fittings, closure position and construction, but verify the required dimensions against the machine. A worn sample may be stretched, distorted or incomplete. Its apparent maximum reach does not establish an acceptable working length.

Photograph both mountings, the complete installation and any failure areas. Mark measured values, drawing values and estimates separately. Where a dimension cannot be checked safely, identify it as unverified instead of presenting it as a confirmed production dimension.

Questions to Resolve Before Requesting a Quote

Is machine stroke enough to specify the bellows?

No. The same stroke can occur between different mounting distances. Both installed length limits, the cross-section and end fittings are needed to establish whether a proposed cover fits.

Can the compressed length be calculated from fold count?

Not reliably from fold count alone. Fabric thickness, seams, reinforcement, closure construction and end transitions affect the folded package. Provide the available space and request a confirmed design.

Should extra length be added before sending the measurements?

Send the actual measured distances first. Identify any required additional movement separately, and have allowances agreed during design review. Unexplained additions can obscure the installation requirements.

Send a Measurement Package for Custom Sewn Bellows

A useful quotation package contains a dimensioned side view, end views for both mountings, photographs of the installation and the required operating conditions. Include Lc and Le with their reference points, the protected component envelope, outside space limits, mounting details, closure access and any support requirements.

State the quantity, whether the project is a replacement or new OEM assembly, and any drawing revision or material requirement. BSTFLEX can evaluate drawings, measurements or an existing sample for a custom sewn bellows proposal. Confirm the final dimensions and construction on the approval drawing before production.

Request a custom sewn bellows quotation with your drawing and application details, or email sales@pkfiresleeve.com.