How to Measure and Specify Scraper Seals for Replacement
Scraper seals (also called wipers or dust rings) protect hydraulic and pneumatic systems by excluding contaminants and retaining lubricant. Replacing them requires accurate measurement, correct groove and shaft assessment, and the right material choice for pressure, temperature, and media. This article walks through practical measurement steps, specification checks, material selection, and procurement details to ensure reliable replacement of scraper seals with verifiable, industry-aligned methods. References to general sealing principles are included for context (Wikipedia: Seal (mechanical)).
Understanding scraper seals and why precise measurement matters
What scraper seals do and where they fit
Scraper seals — also often called wipers, dust rings or lip scrapers — are installed on cylinder rods or piston rods ahead of the primary dynamic seals. Their job is to remove dirt, water, and abrasive particles from the moving rod surface before contaminants can reach the rod seal or cylinder bore. Because they operate in the contamination interface, even small dimensional errors or inappropriate material choices drastically shorten system life.
Common failure modes tied to incorrect sizing
Typical failures include lip tearing, extrusion, rapid wear, and hardening from incompatible fluids. These failures usually stem from: improper radial clearance leading to excessive friction or insufficient wiping; wrong groove depth or width causing lip inversion or extrusion; incorrect material for temperature or chemical exposure; or using a single-lip scraper where a two-lip design would prevent leakage better. Understanding these failure modes helps interpret wear patterns and back-calculate correct dimensions and materials.
Standards and reference materials
General sealing principles and terminology are documented in engineering references and standards bodies (see ISO). Manufacturer catalogs (SKF, Parker, etc.) provide practical fitting tolerances and material performance tables. Use these as verification points when specifying replacements and when in doubt consult the seal supplier's data sheets.
Tools, measurements, and the step-by-step measurement process
Tools you'll need
- Vernier caliper or digital caliper (0.01 mm resolution)
- Micrometer (for shaft diameter and lip thickness)
- Depth gauge or dial indicator (to measure groove depth)
- Profile gauge or simple paper template (for complex cross-sections)
- Flashlight and magnifier (inspect wear patterns)
- Camera and note-taking tools (document orientation and installation features)
Key dimensions to record (with method)
Record these values precisely — they form the basis of the replacement spec:
- Shaft (rod) nominal diameter (D): measure with micrometer at several axial locations; record max and min.
- Outer diameter available for seal (OD): measure bore or housing ID correlating to the scraper OD when installed.
- Groove width (G): use caliper across groove mouths; confirm with depth gauge for lip clearance.
- Groove depth (H): depth gauge from lip seat to groove bottom; confirm against cross-section thickness.
- Axial position/location: distance from reference features (flange, stop, shoulder); photograph and annotate.
- Cross-section profile: copy the removed seal with tracing paper or photograph for the supplier.
Measurement checklist and tips
When measuring, take multiple readings and record min/max. If rods show wear or out-of-roundness, measure at 90° intervals. Note groove edge radii; sharp corners may require chamfering for installation. If the original seal is missing or damaged beyond recognition, use the mating parts (rod and housing) to determine the functional envelope and choose a standard profile that matches the clearance and application.
Material selection, application limits, and specification details
Selecting a material based on environment
Material selection is driven by operating temperature, chemical compatibility, exposure to contaminants, and friction requirements. Typical material choices for scraper seals and wipers include:
| Material | Typical temperature range | Strengths / Notes |
|---|---|---|
| NBR (Nitrile) | -30°C to +110°C | Good oil resistance, common for hydraulic systems |
| FKM (Viton) | -20°C to +200°C | High temp and chemical resistance; good for aggressive fluids |
| Silicone | -60°C to +200°C | Excellent low-temp flexibility, limited abrasion resistance |
| PTFE (filled) | Up to +260°C | Low friction, excellent chemical resistance; often used as a backing or lip filler |
| FFKM | Up to +300°C | High-end chemical and temperature performance (expensive) |
Data aggregated from manufacturer data sheets and general material tables. Always request certification or datasheet from your supplier for critical applications.
Operational limits: pressure, speed and temperature
Scraper seals are not primary pressure-retaining elements; they are optimized for contaminant exclusion and minimal friction. For high-pressure or high-speed systems: confirm the dynamic speed (m/s) and intermittent pressure exposure. Where scrapers see splashback or fluid, consider two-lip designs with an inner sealing lip of compatible material. For guidance on dynamic seal selection and limits consult technical catalogs such as those from major suppliers (e.g., SKF SKF Seals).
Specification template for ordering
Provide suppliers with a clear spec sheet including:
- Application: hydraulic/pneumatic, rod diameter, cylinder bore
- Key dimensions: rod OD, groove width, groove depth, axial location tolerances
- Material and hardness (e.g., NBR 70 Shore A) or request vendor recommendation
- Expected operating conditions: temperature range, max pressure, fluid media, speed (m/s)
- Quantity, part number of original if available, and drawing or photo of installation
Practical examples, verification, and installation tips
Example: measuring a worn rod and specifying a replacement
Case: rod nominal 32 mm, worn pocket at 15 mm from shoulder, original scraper missing. Steps:
- Measure actual rod diameters at multiple points: 31.98–32.02 mm suggests within tolerance; localized wear at 31.70 mm indicates abrasion.
- Measure bore and groove area available for scraper OD (housing ID 42.00 mm, available radial wall 5 mm).
- Choose a scraper profile with a flexible outer lip able to accommodate 0.30 mm localized rod taper; specify material NBR 70A if temperature <110°C and fluid is standard hydraulic oil.
- Include installation notes: chamfer groove edges 0.2 x 45° to ease install, verify radial clearance 0.1–0.3 mm after assembly.
Table: typical groove and clearance guidelines (general reference)
| Parameter | Typical value / guideline | Notes |
|---|---|---|
| Radial clearance (rod to scraper inner diameter) | 0.05–0.30 mm | Smaller clearance improves wiping but increases friction; adjust to speed and material |
| Axial clearance in groove | 0.1–0.4 mm | Allow slight axial movement for lip flexing and dirt ejection |
| Groove depth tolerance | ±0.05–0.15 mm | Depends on seal cross-section and material compressibility |
These values are general guidelines. Always cross-check with the seal supplier's recommended tolerances for the selected profile and material.
Installation and inspection checklist
- Clean rod and bore thoroughly and remove sharp edges or burrs; chamfer as necessary.
- Lubricate the scraper lightly with compatible fluid to ease installation and initial running.
- Ensure correct lip orientation (open lip toward contaminants) before pressing into groove.
- After installation, cycle the rod several times and re-check that the lip sits correctly and there is no extrusion or inverted lip.
- Document serial/lot numbers and supplier datasheets for traceability.
About Polypac and why supplier selection matters
Polypac profile and capabilities
Polypac is a scientific and technical hydraulic seal manufacturer and oil seal supplier specializing in seal production, sealing material development, and customized sealing solutions for special working conditions. Polypac's custom rubber ring and O-ring factory covers an area of more than 10,000 square meters, with a factory space of 8,000 square meters. Our production and testing equipment are among the most advanced in the industry. As one of the largest companies in China dedicated to the production and development of seals, we maintain long-term communication and cooperation with numerous universities and research institutions both domestically and internationally.
Products and experience
Founded in 2008, Polypac began by manufacturing filled PTFE seals, including bronze-filled PTFE, carbon-filled PTFE, graphite PTFE, MoS₂-filled PTFE, and glass-filled PTFE. Today, we have expanded our product line to include O-rings made from various materials such as NBR, FKM, silicone, EPDM, and FFKM. Polypac's main product offerings relevant to scraper sealing include O-Rings, Rod Seals, Piston Seals, End Face Spring Seals, Scraper Seals, Rotary Seals, Back-up Rings, and Dust Rings.
Why Polypac can be a good partner
Polypac combines modern production capacity, in-house material development, and industry collaboration to offer rapid prototyping, custom compound development, and batch traceability. For customers with special working conditions (high temperature, aggressive media, or abrasive environments), Polypac can recommend material and profile modifications based on lab testing and field feedback.
Verification, documentation and final ordering checklist
Before you order
Verify all recorded dimensions and environmental constraints. Send a clear engineering drawing or photograph with annotated dimensions, and request material datasheets, hardness, and a sample if the run is small. For critical systems, request a production sample for a bench test before full delivery.
Quality and traceability
Ask for batch traceability, material certificates (raw polymer certificates), and any testing reports (abrasion, compressibility, aging). For safety-critical or high-value equipment, align on acceptance criteria and inspection plan with the supplier prior to production.
Ordering example (spec summary to give supplier)
- Part: Scraper seal, single-lip, rod scraper
- Rod Ø: 32.00 mm (+/-0.02), Bore ID available: 42.00 mm
- Groove: width 6.5 mm, depth 1.8 mm
- Material: NBR 70A (or FKM if temp >120°C)
- Quantity: 200 pcs; sample required before mass run
- Delivery lead time, acceptable packaging, and inspection requirements
FAQ
Q1: How do I measure a scraper seal when the original is missing?
Measure the mating components (rod diameter and housing ID/groove area), photograph the installation area, and record axial locations relative to fixed references. Provide these to the supplier who can propose a profile that fits the functional envelope.
Q2: What is the difference between a scraper seal and a rod seal?
Scraper seals (wipers) remove contaminants from the rod surface and are not intended to hold system pressure; rod seals provide the primary hydraulic sealing function to retain fluid under pressure.
Q3: Can I use PTFE scrapers on my hydraulic cylinder?
PTFE-based scrapers are excellent for low friction and chemical resistance, but they are less elastic and rely on precise groove tolerances. Filled PTFE constructions (e.g., bronze-filled) can improve wear behavior. Choose PTFE when operating temperatures or chemical exposure exceed elastomer limits.
Q4: What tolerance should I use for radial clearance?
Radial clearance of scraper lip to rod is typically between 0.05 and 0.30 mm depending on speed and material. Lower clearances improve wiping but increase friction; consult your seal supplier for exact values for your selected profile.
Q5: How often should scraper seals be inspected or replaced?
Inspection intervals depend on operating environment. For mobile equipment in dusty environments, inspect during regular service intervals (e.g., every 500–1000 hours). Replace immediately if lips show tearing, deep wear grooves, or permanent deformation.
Q6: Can a worn scraper cause rod seal failure?
Yes. A failed or missing scraper allows abrasive particles to reach the rod seal and cylinder bore, accelerating wear and leading to loss of sealing performance.
Contact and next steps
If you need help measuring, specifying, or sourcing scraper seals, Polypac offers technical consultation, custom profile development, and production for small or large quantities. Contact Polypac to request samples, datasheets, or a quotation for Scraper Seals, O-Rings, Rod Seals, Piston Seals, End Face Spring Seals, Rotary Seals, Back-up Rings, and Dust Rings. For application-specific advice and to share drawings or photos, contact our technical team to ensure the recommended scraper seal meets your system's operational and environmental requirements.
References: General seal principles and terminology: Wikipedia; manufacturer technical catalogs and standards guidance (see SKF Seals).
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