How to Specify and Buy a Seal for a New Application

Selecting a seal for a new application involves much more than finding a component that fits the available space. The seal must work with the fluid, pressure, temperature, movement and surrounding hardware. It must also be manufactured consistently, supplied economically and remain available throughout the working life of the equipment.

For an original equipment manufacturer, the decision can affect:

• Product performance
• Manufacturing costs
• Warranty exposure
• Machine reliability
• Regulatory compliance
• Supply-chain resilience
• Future replacement-parts revenue

The correct question is not simply:

Which seal fits these dimensions?

It is:

Which sealing system will perform reliably, remain commercially viable and support the equipment throughout its working life?

1. Begin With a Non-Disclosure Agreement

The development of a seal for a new application will generally begin with a non-disclosure agreement (NDA).

A meaningful technical review may require the OEM to disclose:

• Machine and assembly drawings
• Unreleased product designs
• Groove and housing dimensions
• Operating conditions
• Expected production volumes
• Known failure modes
• Customer-specific requirements
• Prototype and launch dates
• Target costs
• Aftermarket strategy
• Proprietary test results

The NDA should be in place before detailed drawings, samples or commercially sensitive application information are exchanged.

It should identify:

• The parties permitted to receive the information
• What constitutes confidential information
• The purpose for which it may be used
• Whether information may be shared with factories, material suppliers or subcontractors
• How drawings and electronic files must be stored
• Whether copies and samples must be returned or destroyed
• How long the confidentiality obligations remain in force
• Ownership of information developed during the project
• Restrictions on supplying the resulting component to third parties

An NDA protects confidential information, but it does not automatically establish ownership of the resulting seal design, tooling, compound or intellectual property. These matters should be addressed separately in the development, tooling and supply agreements.

Once the NDA is in place, the initial technical and commercial discussion can begin.

2. Start With a General Discussion With Sealparts

The process will normally begin with a general discussion between the customer and Sealparts in the UK.

At this stage, the customer does not necessarily need to have a complete seal specification. The purpose of the first discussion is to understand:

• What the machine or assembly must do
• What is being sealed
• Whether the application is new or an improvement to an existing design
• The operating environment
• Known technical difficulties
• Prototype and production timescales
• Estimated annual quantities
• Commercial and supply-chain priorities
• Long-term replacement requirements

This early conversation can take place before every engineering detail has been finalised. In fact, involving Sealparts before the seal housing and metalwork are fixed can create more opportunities to use proven profiles and established housing dimensions.

Sealparts will then discuss the application with the most appropriate manufacturing partner. This is a completely transparent relationship. The customer knows which manufacturer is being considered, where the component is expected to be produced and how the technical and commercial supply chain is structured.

Where required, Sealparts can involve:

• Seal technicians
• Application engineers
• Material specialists
• Production engineers
• Tooling engineers
• Quality and compliance personnel

This allows the application to be examined by the people responsible for designing, manufacturing and validating the component.

Sealparts remains the UK point of contact, helping to:

• Coordinate the technical discussion
• Present drawings and application data
• Resolve questions
• Manage quotations
• Arrange samples and prototypes
• Clarify country of origin and lead times
• Coordinate production and supply
• Maintain the commercial relationship

The objective is not to place a barrier between the customer and the manufacturer. It is to create a clear connection between the UK customer, Sealparts and the most appropriate manufacturing expertise.

3. Define What Must Be Sealed

The technical process begins by identifying the operating medium. This might include:

• Mineral hydraulic oil
• Synthetic hydraulic fluid
• Water or water-glycol
• Compressed air
• Lubricating oil
• Automatic transmission fluid
• Fuel
• Refrigerant
• Solvent or process chemical
• Food, pharmaceutical or potable-water products

Provide the exact fluid designation wherever possible. Descriptions such as "hydraulic oil" or "chemical" may not contain enough information to confirm compatibility.

The seal supplier should also know about every substance the component may encounter, including:

• Lubricants used during assembly
• Cleaning fluids
• Flushing agents
• Process chemicals
• Preservatives
• External contamination
• Alternative fluids introduced during servicing

A material compatible with the principal operating fluid may still be damaged by a cleaning chemical, assembly lubricant or maintenance product.

4. Establish Whether the Application Is Static or Dynamic

A static seal operates between surfaces that do not move relative to one another. A dynamic seal works against a moving surface.

For a dynamic application, identify the form of movement:

• Reciprocating
• Rotary
• Oscillating
• Slow adjustment
• Intermittent cycling
• Continuous operation

This helps determine whether the application requires an O-ring, hydraulic packing, pneumatic seal, rotary shaft seal, spring-energised seal or another specialist profile.

An O-ring may be suitable for one static application but entirely inappropriate for a high-cycle reciprocating duty with poor lubrication.

5. Identify the Sealing Position

For hydraulic and pneumatic equipment, define the function of each component within the complete sealing arrangement.

This may include:

• Rod seal
• Piston seal
• Wiper or scraper
• Buffer seal
• Static housing seal
• Cushion seal
• Wear ring
• Guide ring
• Rotary shaft seal

State whether the seal is single-acting or double-acting and identify the direction of pressure.

The sealing arrangement must be considered as a complete system. A rod seal cannot compensate indefinitely for inadequate guidance, a damaged rod, excessive clearance or an unsuitable wiper.

6. Define the Pressure Conditions

Provide more than the nominal system pressure. The specification should include:

• Normal operating pressure
• Maximum continuous pressure
• Temporary pressure peaks
• Frequency and duration of pressure peaks
• Pressure direction
• Pressure reversals
• Possibility of vacuum
• Potential for trapped pressure between seals
• Hardware and extrusion clearances

A seal operating continuously at 100 bar presents a different challenge from one normally operating at 10 bar but repeatedly exposed to sharp peaks of 100 bar.

Material hardness, operating temperature, seal geometry and extrusion clearance must be considered together. Pressure figures alone do not establish whether a seal will survive the application.

7. Establish the Temperature Range

Provide three temperature figures wherever possible:

• Minimum start-up or ambient temperature
• Normal operating temperature
• Maximum temperature, including temporary peaks

The temperature experienced at the sealing interface may be higher than the general fluid temperature. Friction, speed and restricted lubrication can create localised heat.

Temperature can affect:

• Hardness
• Elasticity
• Compression set
• Chemical resistance
• Extrusion resistance
• Friction
• Wear rate
• Expected service life

A material may tolerate a stated maximum temperature for a short period without being suitable for continuous operation at that temperature.

8. Define Movement, Speed and Duty Cycle

For reciprocating applications, provide:

• Stroke length
• Rod or piston speed
• Number of cycles
• Cycle frequency
• Acceleration and deceleration
• Whether movement is continuous or intermittent
• Length of static periods
• Required positioning accuracy
• Acceptable friction
• Sensitivity to stick-slip

For rotary applications, provide:

• Shaft diameter
• Rotational speed
• Direction of rotation
• Continuous or intermittent duty
• Runout and eccentricity
• Lubrication conditions

Surface speed can then be calculated from shaft diameter and rotational speed. A seal suitable for slow hydraulic movement may not be suitable for a rapidly cycling pneumatic cylinder or continuously rotating shaft.

9. Provide the Hardware Dimensions

Whenever possible, the seal should be selected using the dimensions of the metal housing rather than measurements taken only from a used seal. An old seal may have:

• Swollen
• Shrunk
• Worn
• Hardened
• Taken a compression set
• Become distorted during removal

Important dimensions can include:

• Rod or shaft diameter
• Cylinder bore
• Groove diameter
• Groove width
• Housing bore
• Extrusion clearance
• Lead-in chamfers
• Available installation space

Dimensional tolerances are just as important as nominal measurements.

For a genuinely new design, there is usually an advantage in designing around an established seal profile and recognised housing dimensions. This can reduce tooling costs, shorten lead times and make future supply easier. However, standardisation is not always the correct commercial decision. Its effect on the replacement-parts market must also be considered.

10. Specify the Mating Surfaces

The metalwork is part of the sealing system. The supplier may need to know:

• Surface material
• Surface hardness
• Coating or plating
• Surface roughness
• Direction of machining marks
• Concentricity
• Runout
• Alignment
• Potential side loading
• Quality of the lead-in chamfers

A correctly selected seal may fail quickly against a scratched, misaligned, excessively rough or inadequately supported surface. Persistent rod-seal leakage, for example, may originate from bearing wear, side loading or rod damage rather than from the seal itself.

11. Assess Contamination and the Operating Environment

Identify anything that may reach the sealing interface:

• Dust
• Mud
• Water
• Ice
• Metal particles
• Process debris
• Abrasive material
• Outdoor weather
• High-pressure washing
• Aggressive cleaning chemicals

This information helps determine the correct wiper, scraper and exclusion arrangement. Contamination must be considered across the complete system. Installing a more expensive pressure seal will not necessarily improve reliability if abrasive particles are being drawn past an unsuitable wiper.

12. Select the Material

The material should be selected only after the operating conditions are understood.

NBR

Nitrile rubber is widely used with mineral oils in general hydraulic and pneumatic applications. It is economical and available in many compounds and hardness grades.

FKM

Fluoroelastomer is often selected for elevated temperatures, fuels, oils and demanding chemical environments. However, FKM is not automatically superior to NBR or polyurethane in every dynamic application.

Polyurethane

Polyurethane is widely used for hydraulic rod and piston seals because of its wear, abrasion and extrusion resistance. Different polyurethane chemistries can behave differently in water, heat and humid environments.

EPDM

EPDM is used for selected water, steam, weathering and special-fluid applications. It is generally unsuitable for mineral oils and many hydrocarbon-based lubricants.

PTFE

PTFE compounds provide low friction, broad chemical resistance and wide temperature capability. Because PTFE is not conventionally elastic, it is commonly energised by an O-ring, spring or another resilient component.

Silicone and FVMQ

These materials may be selected for particular temperature ranges, fluids or specialist applications, but their mechanical limitations must also be considered.

The generic polymer description is not a complete material specification. "NBR," "FKM" or "polyurethane" does not define:

• Exact formulation
• Hardness
• Reinforcement
• Cure system
• Low-temperature performance
• Compression set
• Fluid compatibility
• Abrasion resistance
• Manufacturing consistency

For critical applications, request relevant test data for the actual compound being offered — not only a generic polymer data sheet.

13. Define the Required Service Life

The supplier should understand what successful performance means. Consider:

• Required operating hours
• Number of cycles
• Planned maintenance interval
• Permissible leakage
• Acceptable friction
• Shelf-life requirement
• Consequence of failure
• Ease of replacement
• Warranty period
• Cost of machine downtime

A small saving on the unit price can be irrelevant if seal failure stops a production line or requires major machine disassembly.

14. Identify Regulatory and Documentation Requirements

Determine whether the application requires:

• Food-contact compliance
• Potable-water approval
• Pharmaceutical documentation
• Oxygen-service preparation
• Flame resistance
• Explosive-decompression resistance
• Low-temperature qualification
• Certificate of conformity
• Material certification
• Batch traceability
• Country-of-origin documentation
• Restricted-substance declarations
• Customer-specific approval

These requirements should be established before the seal is quoted and certainly before tooling or production begins.

15. Consider Country of Origin and Tariffs

Country of dispatch and country of manufacture are not necessarily the same.

A seal invoiced or shipped by a European distributor may have been manufactured elsewhere. Similarly, an international manufacturer may produce different ranges in different countries.

Sealparts operates transparently, enabling the customer to understand:

• The proposed manufacturer
• Country of manufacture
• Country of dispatch
• Location of moulding or machining
• Location of finishing operations
• Preferential and non-preferential origin
• Commodity or tariff classification
• Supporting origin documentation
• Applicable import duties
• Potential additional trade measures
• Customs and brokerage requirements

The commercial comparison should be based on landed cost:

Product price + tooling + freight + insurance + duty + customs charges + inspection + inventory cost + supply risk.

Tariff classifications, origin rules and rates can change. They should be confirmed for the specific product, manufacturing origin and destination at the time of purchase.

Country of origin may also affect customer approvals, public procurement, sanctions compliance, defence applications and the ability to claim preferential duty treatment.

16. Decide Between a Standard and Custom Seal

Standard catalogue sizes usually provide:

• Lower initial cost
• No dedicated tooling charge
• Faster prototype supply
• Lower minimum quantities
• Greater availability
• More potential sources
• Easier emergency replacement
• Lower obsolescence risk

A custom size or profile may be justified where the application requires:

• A restricted installation envelope
• Improved sealing performance
• Controlled friction
• Reduced leakage
• Simplified assembly
• A proprietary interface
• Longer service life
• Protection of the replacement market

A custom diameter alone may provide little meaningful protection. If the component uses a common profile and material, another supplier may still be able to reproduce it.

A more defensible component may combine controlled geometry, tolerances, material specification, identification, application knowledge and documented ownership. However, every custom element increases dependence on tooling, manufacturing knowledge and long-term supplier continuity.

17. Protect the Replacement and Spares Market

Protecting the spares market is a separate decision from selecting the technically correct seal.

An OEM should decide whether replacement components will be:

• Freely available standard items
• Custom sizes based on recognised profiles
• Fully proprietary components
• Supplied only as part of an authorised seal kit
• Controlled through specific part numbers and packaging

Consider:

• Can the seal be identified from its dimensions?
• Is the profile already widely available?
• Will the component carry a unique part number?
• Can the manufacturer sell it to third parties?
• Can distributors supply it directly to the OEM's customers?
• Will drawings and application information remain confidential?
• How will authorised replacements be identified?
• Will seal kits carry batch and machine references?
• Who controls revised and superseded specifications?
• How long must replacement parts remain available?

A standard seal offers security through multiple potential sources, but provides little control over the aftermarket. A proprietary seal may help protect replacement sales, but creates greater tooling, minimum-order, continuity and obsolescence risks.

The correct balance depends on:

• Machine production volume
• Expected service life
• Value of the spares market
• Cost of equipment downtime
• Importance of authorised servicing
• Risk of counterfeit or unsuitable substitutions
• Ability to maintain long-term stock and documentation

The spares strategy should be agreed during design — not after the equipment has entered service.

18. Establish Ownership of Custom Tooling

Tool ownership should be agreed in writing before a tooling charge is paid. The agreement should identify:

• Legal owner of the tool
• Physical location
• Tool number
• Associated component and drawing
• Whether payment grants full ownership
• Whether it is only a contribution to tooling costs
• Who is responsible for maintenance
• Expected tool life
• Storage conditions
• Insurance responsibility
• Whether the tool can be inspected
• Whether it can be transferred
• Who pays removal and transport costs
• What happens if the supplier closes or relocates
• What happens following fire, flood or another loss
• Whether the supplier may subcontract production
• Whether the tool may be used for another customer
• Conditions governing modification or destruction

Paying a tooling invoice does not necessarily provide practical control of the tooling or an unrestricted right to remove it.

The purchasing agreement should explicitly state that dedicated customer-funded tooling:

• Remains the customer's property
• Is permanently identified
• Is separately recorded
• Is maintained in serviceable condition
• Is used only for authorised production
• Cannot be moved, modified, scrapped or used for a third party without written permission

The exact wording should be reviewed in the context of the contract and relevant jurisdiction.

19. Maintain a Tooling Register

For every custom-moulded seal, retain:

• Approved production drawing
• Drawing revision
• Material specification
• Compound reference
• Tool number
• Tool photographs
• Cavity count
• Tool location
• Supplier acknowledgement of ownership
• Tooling invoice and payment record
• Sample inspection report
• Production approval
• Maintenance and repair history
• Last production date
• Written approval for relocation or subcontracting

Tooling should be reconciled periodically with the manufacturer.

A spreadsheet showing where a tool was located several years ago is not sufficient evidence that it remains present, identifiable and usable.

20. Understand the Limits of Tooling Ownership

Owning the physical mould does not necessarily make the product portable. The original manufacturer may still control:

• Compound formulation
• Material mixing
• Cure system
• Shrinkage allowances
• Moulding parameters
• Post-curing
• Surface treatment
• Inspection methods
• Specialist production knowledge

The tool may also have been designed around a particular press, process and compound shrinkage rate. Moving it to another manufacturer may not produce an identical component — or any usable component at all.

For important proprietary seals, establish whether the OEM owns only the physical tool or has enough controlled technical information to reproduce the approved part through an alternative source.

21. Agree Intellectual Property and Drawing Ownership

The NDA protects confidential information, but the project also needs clear rules concerning intellectual property. The parties should agree:

• Who owns the original application drawing
• Who owns the final seal drawing
• Who owns modifications proposed by the manufacturer
• Who owns the groove design
• Who owns the production tooling
• Whether the seal profile is proprietary or already established
• Whether the manufacturer may reuse the design
• Whether the customer can transfer the design to another supplier
• Who controls drawing revisions
• Which document represents the approved specification

A customer-funded development project does not automatically mean that every element of the resulting product belongs to the customer. Equally, receiving an OEM's assembly drawing does not permit a supplier or manufacturer to reuse that information for another customer. These boundaries should be agreed before development work begins.

22. Plan for Long-Term Supply and Obsolescence

Machinery may remain in service for decades after original production has ended. Before approving a custom seal, agree:

• Prototype quantities
• Production minimums
• Economic batch quantities
• Annual demand
• Normal lead time
• Emergency lead time
• Raw-material minimums
• Tool-storage period
• Notice before discontinuation
• Last-time-buy arrangements
• Ownership of remaining stock
• Alternative manufacturing routes
• Availability of machined replacements
• Required duration of spares support

A supplier that is competitive during initial production may not remain suitable when annual demand falls from thousands of components to a few replacement kits. The long-term supply plan should therefore include low-volume and end-of-life requirements.

23. Consider Packaging, Identification and Traceability

Packaging is part of the supply specification, particularly where seals may remain in storage before installation. Consider:

• Individual or bulk packaging
• Protection from deformation
• Protection from light and ozone
• Temperature and humidity controls
• Batch identification
• Manufacturing date
• Shelf-life controls
• Customer part number
• Manufacturer part number
• Country-of-origin marking
• Barcode or data-matrix requirements
• Kit identification
• Installation instructions

For proprietary replacement kits, controlled packaging and identification can help distinguish authorised components from unknown substitutes.

Traceability should be sufficient to connect a supplied component to its material batch, production batch, inspection records and approved drawing revision where the application requires it.

24. Compare the Complete Commercial Position

The lowest quoted unit price is not necessarily the lowest-cost option. Compare:

• Unit price
• Tooling cost
• Prototype cost
• Minimum order quantity
• Economic batch quantity
• Freight
• Import duty
• Customs and brokerage charges
• Inspection requirements
• Currency exposure
• Payment terms
• Lead time
• Inventory requirement
• Expected reject rate
• Warranty exposure
• Cost of downtime
• Future spares value
• Supply-interruption risk

A seal costing slightly more from an established and documented supply chain may represent a substantially lower commercial risk.

25. Test the Application Before Production Approval

Catalogue ratings and material data are useful selection tools, but they do not guarantee performance in the completed machine.

A new application will normally progress through:

1. Non-disclosure agreement
2. Initial discussion with Sealparts
3. Technical review with the appropriate manufacturing partner
4. Preliminary profile and material selection
5. Prototype or sample production
6. Dimensional inspection
7. Installation assessment
8. Application testing
9. Examination of used components
10. Design adjustment where required
11. Final production approval
12. Controlled production supply

Testing should reproduce the actual fluid, temperature, pressure, speed, contamination and duty cycle as closely as practical.

For critical applications, the approval process should define measurable acceptance criteria rather than relying only on the absence of an immediate visible leak.

Information to Include With a New Seal Enquiry

Provide as much of the following as possible:

• Signed NDA where required
• Assembly drawing
• Seal-housing drawing
• Rod, shaft, bore and groove dimensions
• Dimensional tolerances
• Static, reciprocating or rotary duty
• Exact operating fluid
• Cleaning and secondary fluids
• Minimum, normal and maximum temperature
• Normal and peak pressure
• Speed, stroke and cycle rate
• Surface material, hardness and finish
• External contamination
• Required service life
• Leakage and friction limits
• Regulatory requirements
• Traceability requirements
• Intended or permitted country of manufacture
• Destination country
• Prototype quantity
• Annual production quantity
• Expected spares demand
• Required delivery date
• Tooling ownership requirements
• Intellectual-property requirements
• Aftermarket and distribution restrictions

Do not delay the initial conversation simply because all this information is not yet available. Sealparts can help identify which details are required and which questions need to be answered.

A Seal Creates a Long-Term Supply Chain

A new sealing component should be assessed across four connected areas.

Application engineering

Will it seal reliably under the actual operating conditions?

Manufacturing control

Can the chosen manufacturing partner produce the same approved component consistently?

Supply-chain resilience

Can the seal be obtained economically throughout the production and service life of the equipment?

Commercial control

Who owns the drawings, tooling, specification, production knowledge and replacement market?

The lowest-priced seal is not necessarily the lowest-cost solution.

A component that saves a small amount during initial production may create future costs through import duties, minimum quantities, unavailable tooling, uncontrolled substitutions, premature failures or loss of the aftermarket. The best time to address these risks is before the seal, groove and supply arrangement are approved.

Talk to Sealparts About a New Application

Sealparts has supplied hydraulic, pneumatic, rotary and static sealing components since 1980.

The process can begin with a general discussion. Once any necessary NDA is in place, Sealparts can review the application and involve the appropriate technicians, application engineers and manufacturing specialists.

Our approach is transparent. Customers can understand:

• Which manufacturing partner is involved
• Where the seal will be produced
• How the material and profile are being selected
• What tooling may be required
• What the expected supply chain will look like
• How origin, tariffs and long-term spares requirements will be managed

To begin the discussion, send us whatever information is currently available, including drawings, operating conditions, estimated quantities and timescales.

Contact info@sealparts.co.uk to discuss a new sealing application.