SF-1 PTFE Split Bushings for Web-Tension Dancer-Arm Pivots
For an SF-1 PTFE composite split bushing in a web-tension dancer-arm pivot, select against resisting torque and repeatability as well as bearing load. A pivot that carries the arm safely can still introduce unacceptable tension hysteresis. Define the lowest working tension, arm geometry, radial reactions and breakaway-torque acceptance before ordering a replacement sleeve.
The component discussed here is the oscillating arm pivot, not the continuously rotating dancer roller bearing. Paper, film, foil and textile web systems can have different tension sensitivity. This is a drawing-review framework for an existing plain-bush arrangement or an approved design study; it does not claim BONIU has validated a dancer assembly.
Start with the dancer’s torque budget
Montalvo’s comparison of dancer and load-cell control stresses the importance of low mass, low inertia and low friction in a dancer. Its controller discussion is application context, not a specification for an SF-1 bearing.
[1]At a given arm position, the web tensions, gravity, loading cylinder or spring, pivot friction and any dynamic effects contribute moments about the pivot. For a low-tension web, a relatively small resisting moment may matter even when the calculated contact pressure is modest. A position sensor reports arm movement; it does not independently prove that web tension is correct.
Ask the machine designer for the allowable parasitic torque over the arm’s travel. Separate steady sliding torque, breakaway after dwell, direction-dependent hysteresis and the effects of seals, cables and axial retainers. These quantities give a supplier a measurable acceptance target.
Translate pivot friction into a tension sensitivity
The following original example is a quasi-static sensitivity calculation. Every numerical input is assumed, not a BONIU measurement or an allowable operating limit. Suppose two identical-radius journals have a combined radial-reaction magnitude of 300 N, a sliding diameter of 20 mm and an assumed friction coefficient of 0.08.
| Quantity | Assumption or calculation | Result |
|---|---|---|
| Combined journal friction torque | M = μ × ΣR × d/2 = 0.08 × 300 × 0.010 | 0.24 N·m |
| Effective web-torque leverage | G = ∂Mweb/∂T, assumed at this arm position | 0.40 m |
| One-direction tension offset magnitude | ΔT = M/G | 0.60 N |
| Idealised reversal band | 2M/G, assuming symmetric Coulomb friction | 1.20 N |
G is the sum of the web-force moment contributions per unit tension for the actual web path. It is not automatically the arm length or roller radius. Obtain it from the machine geometry at each relevant arm position, including changes in wrap and force direction.
The model holds the radial reactions fixed and excludes inertia, seal drag, roller-bearing losses and other torques. Actual reactions and breakaway friction may vary with position and load. Use the calculation to set questions and test conditions; do not use it to predict control accuracy or to certify a tension range.
Compare bearing arrangements against the control requirement
| Arrangement | Potential review point | Evidence before release |
|---|---|---|
| SF-1-style steel-backed PTFE split bush | Compact plain sliding interface; friction depends on the installed system | Exact formulation, counterface specification, measured torque envelope and installed clearance. |
| Greased metallic or POM-lined plain pivot | Lubricant condition and dwell can affect drag | Approved grease, supply method, temperature and torque after rest. |
| Rolling-element pivot | Different friction, clearance and support architecture | Specified bearing arrangement, seals, preload or clearance, mounting and torque test. |
| Flexure or another specialised pivot | May constrain motion differently | Permitted travel, stiffness, fatigue and machine-level design approval. |
These are engineering comparison questions, not a ranking or a recommendation to convert an existing arrangement. Keep the original architecture unless the machine designer approves a change.
Specify the liner and counterface together
GGB’s DU page describes its own steel-backed, porous-bronze, PTFE-and-lead construction and identifies DU as a protected product name. A generic SF-1 label does not establish that formulation, certification or friction performance.
[2]SKF’s composite-bearing catalogue explains that friction depends on load, speed, temperature, mating-surface condition, contamination and lubrication. This supports evaluating the complete interface rather than assigning one constant coefficient to every PTFE composite.
[3]Request the actual sliding mixture and backing, any lead-content restriction, exterior protection and approved lubrication condition. Specify shaft material, hardness, finish and coating. Avoid an unsupported numerical roughness or fit copied from a different proprietary material. The bore should not be reamed simply to reduce torque unless the supplied grade explicitly permits that process.
Prevent assembly drag and define a useful acceptance test
Show both pivot supports on the assembly drawing, their common datum and support span. Inspect coaxiality, shaft straightness and the bores after pressing. Confirm that the shoulders, retaining rings and fasteners do not clamp an unintended axial sliding face. If a flange carries thrust, include its contact and torque separately.
Use aligned press tooling and protect the thin sliding layer from burrs and shaft-entry damage. State the inspection temperature and housing material. Check lint, adhesive dust, wash fluids and corrosion exposure; an exposed sleeve edge can be part of the environmental limit.
For a first article, agree a slow forward-and-reverse torque trace over the required arm travel at specified loads. Include a defined rest period, relevant hot and cold conditions, the intended lubricant state and representative seals. Record the test speed, load setup, instrument resolution and acceptance limits. A free hand rotation with the arm unloaded cannot demonstrate loaded tension sensitivity.
If tension is unstable, inspect the complete mechanism and control loop. Bearing drag is one possible contributor alongside loading-cylinder friction, web-path changes, roller condition, sensor behaviour and controller tuning. Keep the measured symptom separate from a proposed material remedy.
Frequently asked questions
Can an SF-1 bushing be selected from a low friction coefficient alone?
No. The useful acceptance quantity is the complete pivot’s resisting torque under the actual radial and axial loads, after the specified dwell and environmental exposure. A catalogue coefficient does not include every source of drag.
Does this recommend replacing a dancer’s ball bearings?
No conversion is established. A plain-bush pivot can be reviewed where the machine drawing specifies it or the designer approves it. A sensitive low-tension system may require a different bearing arrangement.
Is an SF-1 sleeve the same as GGB DU?
No. DU® is GGB’s protected product designation. Request the supplied SF-1 material construction, formulation and documentation rather than assuming brand or performance equivalence.
Can the same fit be used for both pivot supports?
Both supports need a coordinated datum and tolerance plan. Check their coaxiality, the installed bores and shaft straightness; two individually acceptable bores can still bind an assembled shaft.
Request a drawing review
Send the pivot assembly, sleeve drawing, shaft and housing limits, radial and axial reactions by arm position, web path and lowest tension, effective torque leverage, travel and dwell, environment, allowable resisting torque, quantity and inspection requirements.
Send your drawing for review or email sales@boniubushing.com. BONIU reviews manufacturing feasibility, material availability, documentation and quotation scope for the particular enquiry. This guide does not confirm stock, a certified material batch or approval for a named machine.
Technical references
- Montalvo: dancer or load cells. Dancer-system friction and inertia considerations; not a bearing-material approval.
- GGB: DU metal-polymer bearings. Proprietary material construction and designation.
- SKF: composite dry sliding bearings, catalogue 6110 EN. Friction factors, counterface and assembly guidance for SKF materials; not BONIU ratings.
The cited manufacturers describe their own products and material references. Their ratings, tests and certifications are not BONIU specifications. The application analysis, calculations and enquiry workflow here are engineering guidance; final limits require review of the actual assembly.