Buying O-rings: from stock and with no minimum order quantity
What matters when purchasing O-rings: the right order specifications, the right material selection, and a procurement route with no minimum order quantities and no tooling costs.
- Complete order specifications comprising dimensions (ID x C/S) and material make the delivery unambiguous and prevent disputes.
- Cord thickness (C/S) is the defining dimension, complemented by the inner diameter (ID); ISO 3601 also sets out the tolerances.
- The material determines service life: FFKM, FEP, FKM, NBR and EPDM differ in temperature and media resistance.
- Typical purchasing hurdles include availability, minimum order quantities, tooling costs and gaps in the product range.
- At NH O-RING, you order with no minimum quantity, no tooling costs and a dedicated point of contact; custom sizes are made in 5-6 weeks.
What matters when purchasing O-rings
O-rings are inconspicuous technical components, right up until they fail unexpectedly and a replacement has to be sourced at short notice. Then expensive machinery comes to a standstill, hazardous fluids leak out, or sensitive products become contaminated. The low price level of O-rings makes it easy to overlook how important they are.
A holistic approach to purchasing helps reconcile technical requirements with long-term cost-effectiveness: moving away from a narrow focus on unit price and toward a life-cycle view. This is the most efficient approach, especially for demanding applications.
For this to work in practice, buyers, manufacturers and distributors need to speak the same language. That starts with the technical properties of the O-rings. Complete order specifications leave no doubt as to which O-ring is to be supplied. This creates clarity, enables fast logistics processes and prevents disputes. The focus is primarily on the O-ring's dimensions and its elastomer material.
Order specifications when purchasing O-rings
During design, the compression of the O-ring in the installed position is defined. It results from the distance between the two sealing faces and the O-ring's cord thickness. This cord thickness, also called the cross-section, is slightly deformed under pressure so that the material gains more contact area on the components. This also helps seal components that are slightly eccentric to one another due to tolerances.
Accordingly, cord thickness is the defining dimension of O-rings. Added to this is the inner diameter. It is often chosen so that the O-ring is slightly stretched when it sits in a groove on the inner part of the seal. These definitions give rise to the dimensions, generally stated in the format ID x C/S: ID stands for the inner diameter, C/S for the cross-section or cord thickness.
| Specification | Meaning |
|---|---|
| ID | Inner diameter of the O-ring |
| C/S | Cross-section or cord thickness, the defining dimension |
| Material | Base elastomer and, where applicable, compound |
| Standard | Reference to ISO 3601 for dimensions and tolerances |
The globally recognized standard ISO 3601 sets out this methodology. It also contains a dimension list that defines many O-rings in standard sizes of inner diameter and cord thickness. However, the designation system can also be applied to any dimensions that do not conform to the standard. Sizes are commonly given in inches or millimetres.
When an order references ISO 3601, the permissible tolerances are agreed at the same time, since the standard specifies maximum values for dimensional deviation depending on cord thickness. Our guide on measuring O-rings shows how to correctly take the dimensions of an installed O-ring.
Selecting the right O-ring material
The dimensions determine whether the O-ring undergoes the necessary deformation in the installed position. The right material selection ensures that it permanently retains its shape and elastic properties. In operation, O-rings are exposed to temperatures, friction and a wide range of chemicals.
Media can migrate into the O-ring and cause it to swell. Likewise, components can be leached out, causing the O-ring to shrink and lose elasticity or hardness. These properties are in turn essential to the sealing effect. Common O-ring materials are based on various elastomers, each with specific properties and media resistances.
The better an O-ring's material is matched to the real operating conditions, the longer the service life that can be achieved. In addition to the base material, purchasing can also specify particular material blends that are even better matched to specific requirements. Details can be found in the manufacturers' own data sheets. Our articles on FFKM O-rings and FKM O-rings offer a more in-depth look.
Challenges when purchasing O-rings
Standardized dimensions and materials make it possible to describe O-rings unambiguously. This makes collaboration easier for distributors and buyers and helps prevent misunderstandings. In practice, however, additional hurdles often arise.
Availability
Standard-compliant O-rings are held directly in stock in many sizes; that's standard practice among O-ring suppliers. Taken seriously, this places high demands on inventory management. The ISO 3601 dimension table specifies several cord thicknesses and far more inner diameters, multiplied by the large number of different materials and individual compounds. On top of that come non-standard O-rings in metric dimensions, inch-sized O-rings and special sizes. The more specific the requirement, the more limited the availability. Very rare special sizes sometimes cannot be supplied from stock at all and instead have to be produced in a time-consuming special process.
Our solution: We supply nothing but O-rings, so our warehouse is correspondingly full of O-ring sizes. Should an O-ring ever not be in stock, we produce any quantity and size for you within 5-6 weeks.
Minimum order quantity
To achieve a better ratio of logistics effort to revenue given the low unit prices, many suppliers rely on minimum order quantities. These are often 5 or 10 pieces, though the minimum can also be larger. For buyers, this raises the question of how to properly store the O-rings that aren't currently needed.
Our solution: With us, you order completely free of minimum quantities or minimum order values.
Tooling costs
O-rings in special sizes generally require dedicated tooling to be made. The predominant manufacturing processes are injection moulding and compression moulding. Both produce O-rings with properties that are consistent throughout the entire part, but both require a tool that gives the raw material its final shape. Tooling costs for compression moulding are lower than for injection moulding, but this remains cost-intensive for small quantities. An alternative is splice vulcanizing, in which O-rings are joined from continuous-length cord without tooling, resulting in a marked reduction in mechanical load-bearing capacity.
Our solution: With us, you order O-rings completely free of tooling costs. If a tool is needed for a given size, we take care of having it made.
Gaps in the product range
One component, one partner: purchasing wants to reduce complexity and cover its needs through a small number of suppliers. This makes collaboration simpler and usually more profitable. Gaps in the range make that difficult. If certain materials or sizes are missing, buyers have to look elsewhere and spread the volume across several suppliers. This increases the effort involved in quality assurance and supplier management.
Our solution: We supply nothing but O-rings, and we do it really well. You get every material in almost any size, quantity, colour, hardness and approval. Our O-ring range gives you an overview.
How to request O-rings from us
Engineering defines the requirements for the O-rings. But purchasing also has significant leverage: building stable supply relationships, securing the availability of the required O-rings and maintaining high quality are among its most important tasks.
For an inquiry, the dimensions in the format ID x C/S, the desired material, and details of the contact medium, temperature and any approvals required are all you need. You'll receive a quote in record time, with no minimum order quantities and a dedicated point of contact for all your needs.
Looking to order on a regular basis? Become a customer and secure short lead times and dependable availability.
Frequently asked questions
What information is needed to purchase O-rings?
What does the specification ID x C/S mean?
Does NH O-RING have a minimum order quantity?
What does a tool for special sizes cost?
How quickly are O-rings that aren't in stock available?
Which material is right for my application?
