Buying O-Rings

An Industry Insider’s Guide to Sourcing Rubber O-Rings

by Phil Maxwell – March 26, 2005

1. Determine Size(s)

By Standard Specification References

Many already know the size or sizes of their requirement by one of several widely used industry standards like the AS568 specification.  This relates a three digit numeric code (i.e. -108 or 568-108) that defines sizes and tolerances of hundreds of standardized o-ring configurations in either inch or metric millimeter units, though some vendors also the alternate English or Metric values, also.  AS568 is the most commonly used reference the the United States, often being assumed in callouts referring to the “dash” number alone as the o-ring size.  Other standards include British Standard metric sizes, JIS standard sizes, JASO standard sizes, and AS871.

By Application Requirements

If a standard size designation is not available, every effort should be made to match the requirement to such a standards for the sake of cost and availability.  Be sure to check inch/metric cross references if unable to find the size you need on one or the other before resorting to a custom size, which you can expect to cost a lot more.

Other alternatives to consider might be alternately shaped cross-section ring seals, like Q, D, and particularly so-called Square (cross-section) O-Rings, which can be made by an extrusion and lathe cutting process at reasonable prices and tooling costs, though usually accompanying high minimum quantities.   Die cut ring gaskets can also be a very good overall cost option for some applications because of the relatively low initial tooling costs and minimum quantities associated with the process.

Most o-rings are dimensioned by ID (inside diameter) and CS (cross section diameter, width, wall, or thickness) with OD (outside diameter) given as an untoleranced reference dimension.   Some also state nominal fractional dimensions, but these vary from actual and are usually for reference only, whether stated as such or not.

Size is Important

It should be noted that in the case of o-rings, size is very important.  Users who are uncertain of the o-ring dimensions most suited to their application should refer to widely available engineering resources on the subject such as the following:

O-Ring Design Guidelines
Radial O-Ring Selector Calculator

2. Determine Material Genre

By Standard Specification References

Generally speaking, if the requirement is keyed to one or more standardized material specifications (Mil, AMS, FDA, etc.), this is all the information needed.   Established o-ring vendors can generally identify these requirements by their reference number, but pricing is generally only available through special quotation.   O-ring rubber compounds are much more apt to be specified by such standards than by the detailed properties usually included in them, that is, where more than generic specifications pertain.

Some standard specifications are often met by commercial grade compounds, like ASTM D2000 variations and AMS 3304, but only a small portion of vendors cite their products accordingly.

By Application Requirements

Widely published and generally consistent guidelines cover the primary considerations of temperature ranges, chemical (fluid/gas) compatibility, durability, and cost considerations, but only in general terms.

The most common choices are Buna-n (Nitrile Rubber, NBR), which is by far and away the most common and least expensive, plus Silicone, Viton® (Fluoroelastomer), EPDM, Neoprene, and various Perfluoroelastomer compounds like Kalrez®.   These cover the vast majority of o-ring sealing requirements, but not only are there numerous significant minor categories, there are also a wide array of sub-categories and proprietary compounds branching down from each of these general elastomer classes.  Wide variations of physical properties and, thus, performance expectations, can be found within all general categories of elastomers.  In other words, the fact that published information implies that Buna-n is the best rubber of choice for an application doesn’t make all varieties of Buna-n rubber equal, for instance. 

Unless the user doesn’t care how long and well an o-ring will maintain a seal, this isn’t enough information.  It doesn’t have to be too difficult in most cases, but a closer review is in order because better quality o-rings will generally work better, last longer, and don’t necessarily cost more.

Most general purpose commercial rubber o-rings will work fine in static sealing applications that don’t push an elastomer’s temperature and chemical resistance limits.   High pressure static and all dynamic seals require more durable rubber, making properties like tensile strength, ultimate elongation, and resistance to tearing, abrasion, and compression set important features, though not necessarily beyond the limits of standard commercial o-rings.   

Fluid and gas compatibility is the other main prong of concerns to be addressed, and the need for resistance is largely dependent on the nature of the application, also.   The more stress, temperature, and critical the application, the more important it is to use rubber that is compatible with the environment.   Low pressure static seals can often withstand conditions that moderately degrade the rubber without any functional loss, but every effort should be made to use fully compatible rubber seals in high pressure and/or dynamic applications.

It is at this point that you should have a good idea of what you need as far as o-ring size, material class, and whether or not standard commercial grade o-rings are likely to meet your requirement.   Next comes comparing the actual offerings on the market to find the best combination of quality and price for the application. 

See Related: O-Rings on the Web – Detailed website, price, and specification comparisons between leading online o-ring suppliers.

Test, Test, Test

If doubts about the viability of standard varieties exists, you might want to consult with a dealer/manufacturer, but the only guarantee of a seal’s performance in any application is through testing.  No reputable o-ring manufacturer warrants how their products will perform in any particular application, which is why they generally make specific physical property data available instead, whether distributors publish it or not.   The most prevalent concerns relate to fit or degradation the rubber due to contact with fluids, gases, and extreme low or high temperatures.   Because of the unlimited possible combinations of these and other factors, the only final proof of an o-ring seal’s performance is through testing, but thorough research and good information will generally minimize the process.

3. Compare Competing Brands

On Physical Properties

Once fairly clear bearings on the o-ring size and rubber type are attained, it is then time to compare available options.  It is not enough to simply compare prices between same-size o-rings sold merely as “70 Duro Buna-N” or “75 Duro Viton ®,” for example, because these terms only partially define what the actual products are.   We recommend comparing the basic physical property specifications mentioned above, bypassing those who don’t state at least durometer, tensile strength, and ultimate elongation property specs.   Make sure the properties they state really are presented as specifications for the products being offered for sale, and not just general properties of the rubber class or even actual test data that doesn’t include toleranced values explicitly stated as pertaining to their o-rings.

Beware of o-ring vendors who merely state the durometer or even more complete information like tensile strength and elongation without specific tolerance ranges (+/-, minimum, maximum, or range); for instance, there is a huge difference between a tensile strength value stated as “2,000 psi” and one stated as “2,000 psi minimum,” etc.   The latter obliges the vendor to meet specific quality levels, whereas the former lends no assurance of anything regarding the actual products offered for sale.  No stated minimum, maximum, or tolerance range indicates reference or information only, not precisely as stated or +/- 0.  Just keep in mind that you cannot hold a vendor liable for quality features that are not precisely stated.  Even actual laboratory test data does not necessarily constitute a quality guarantee.

Do not be deterred by minor variances in physical properties unless maximizing resistance to compression, abrasion, etc. are important concerns.  Most minor differences won’t adversely affect performance in most applications, though for the sake of assuring consistent quality and performance, it is essential that the factors be considered.   In other words, don’t save a few pennies buying o-rings of uncertain or clearly substandard quality, but, conversely, don’t pay a bunch more for a little higher tensile strength or ultimate elongation unless your application really requires it.

On Prices

In addition to incomplete and downright misleading information on standard quality measures for o-rings, there is also a serious lack of pricing information published openly by o-ring sellers.  Aside from those listing no prices at all (over half), many sources only make their prices available through cumbersome online ordering systems that offer no price breaks, discounts, or even the ability to see more than single item pricing at a time.  Others list only minimum quantities and prices, often packaged with solicitations for special quotations on higher quantities.  All these approaches are quite unnecessarily user-unfriendly — common, largely standardized commodities like rubber o-rings shouldn’t be sold like used cars for the highest price the salesperson can get away with, especially when others are completely forthcoming in publishing their price and discount schedules (especially those with more competitive pricing overall).

Another peculiar manner of pricing arises from vendors setting standard quantity breaks on all o-rings, regardless of whether the base price is less than a penny or more than a three dollars each.  One vendor offers a minimum purchase cost per item beginning at a little over a dollar on some, but more than $400 on others, and first-tier discount pricing ranging from $12.50 to over $19,000.  Mostly, these pricing structures are so out of step with normal economic considerations, they mix relatively good deals on some items with outrageously high prices on others, depending on the quantity.

If you are looking for minimal quantities, several online suppliers offer minimums from around $1.00 – $5.00 per item with widely varying quantities.  A little shopping can get you several times the quantity for around the same cost.  Prices tend to get more consistent between competitors in higher quantities, but few actually list their price-breaks and discounts.

Shipping, Handling, and Other Potential Surprises

Just like prices, discounts, and physical properties, there is no reason for shipping and handling charges, as well as return policies, not to be published.  Vendors know that consumers can’t possibly measure their costs without knowing these details, so failure to do so obstructs their ability to comparison shop.

4. The Ideal O-Ring Supplier…

Gives all the information you need to compare their products with their competitors and then lets you, the consumer, decide what to buy from whom.   You shouldn’t have to ask for basic physical properties of any commercial rubber product, nor should you have to request a quote to know what standard o-rings will cost in various quantities.   We recommend leaning towards vendors who are more forthcoming and thorough in providing such information not only because of the convenience of their service, but also for what such things imply of their business practices.

Basic components of readily available information that consumers should insist upon include:

  1. Fully detailed dimensions and tolerances of their products, preferably including cross-reference dimensions in alternate units (inch or metric).   If they don’t say both the precise dimensions and the +/- tolerances, move on; you have no guaranteed recourse in the event you get wrong-sized parts.
  2. Basic physical properties including, at a minimum:

    A. Material Class (i.e. Buna-n, EPDM, etc.);
    B. Color (even if you don’t care, they should say);
    C. Hardness (or durometer) including +/- tolerance;
    D. Tensile Strength including tolerance, which is usually stated as a minimum.
    E. Elongation, including tolerances, which is normally stated as minimum “Ultimate Elongation” percentage.

    We recommend against buying from vendors who don’t list such criteria on products they offer for sale, or, at least, obtaining written documentation of the same before buying.

  3. Fully detailed prices, price-breaks, and discounts.   Most that list any prices at all either make viewing them extremely difficult, have absurdly structured discounts, and/or only offer price breaks by special quotation.   Very few publish easily viewed prices and reasonably established quantity-related price breaks, and even fewer offer both item-quantity and overall volume discounts.  In any event, we strongly recommend against buying much more than minimum quantities or even minimum quantities on numerous items without soliciting competitive bids.
  4. Shipping terms, costs, and return policies should also be clearly explained so buyers can compare overall costs between options, something many vendors don’t seem too motivated to enable any more than necessary.

The above information represents nothing more than what any reputable commercial rubber products vendor should have readily available, but, more to the point, it is nothing more than the basic information necessary for potential customers to measure the products being offered for sale.   Thus, even though very few vendors provide it, we think the bar should be set according to what customers need and should, rightfully, expect from any commercial products supplier — that they be forthcoming in what they are offering at what price and terms before they solicit your order.

Phil Maxwell is Vice-President and General Manager of Metro Industries, Inc., parent company of Standard Gasket Products, a Kansas City, Missouri, based distributor of o-rings and other standard rubber gasket and sealing products.