How to Measure Bolt Circle Diameter

Two styles of gasket are used for pipe flanges: full face gaskets and ring gaskets. Full face gaskets need holes for the flange bolts to pass through. When you order this type of gasket the manufacturer needs to know the diameter of the bolt circle. Here we’ll explain what that is and how to measure it. First though, an explanation of the two gasket styles.

Gasket Styles

A full face gasket has roughly the same outside diameter as the flanges being joined. The inside diameter is of course equal to the bore of the pipe. Flange bolt holes in the gasket simplify assembly as the bolts hold the gasket in place and stop it intruding into the flow. The downside is that all the bolts must come out when installing a new gasket.

In contrast, a ring face gasket sits inside the bolts and can be replaced without completely disassembling the joint. They also have a smaller outside diameter, which saves material. They are used principally on raised face flanges where the sealing surfaces are inside the bolt hole diameter.

A full face gasket can be used with raised face flange. Likewise, ring gaskets can be used on flat face flanges, providing the impact of the reduced gasket width is considered.

Bolt Circle Diameter Measurement

The bolt circle is defined as a circle running through the centers of all the bolt holes. As these points are in space they can’t be measured directly. However, there is way.

The bolt holes will all be the same diameter and arranged in diametrically opposite pairs. Looking at the flange end-on, measure from the left edge of the leftmost hole to the left edge of the hole diametrically opposite. (Or measure from right edge to right edge.)

To eliminate any measurement error, repeat on a number of other hole pairs and average the result. This is the number you’ll give your gasket manufacturer, along with inner and outer diameters, number of bolt holes, and the bolt hole diameter. With this information they’ll cut the gasket to the size you need.

What is a Mechanical Seal?

Like gaskets, a mechanical seal stops fluids leaking into places they are not supposed to be. The difference is that gaskets are for static applications like pipe flanges while mechanical seals prevent leaks from around rotating shafts. Hennig Gasket & Seals is in the business of cutting gaskets to shape and supplying gasket material, but we do offer a mechanical seal repair service. Here’s a primer on mechanical seals for those who’d like to know more.

The Shaft-Sealing Challenge

Pumps, compressors, and similar rotating machinery are built around a rotating shaft that passes into or through a housing. Bearings hold the shaft in place but don’t stop the fluid from escaping. Historically, this was done with gland packing material. This looks like rope and is packed around the shaft to prevent leaks.

Gland packing has several limitations:

  • Wears out, requiring adjustment and eventually, replacement
  • Can wear the shaft
  • Creates friction for the motor to overcome

Modern gland materials incorporate PTFE, graphite or other friction-reducing compounds, but the risk of leaks remains. As awareness of the environmental and health and safety consequences has grown, many machinery manufacturers have replaced packing with mechanical seals.

Mechanical Seal Basics

In principle, mechanical seals are straightforward. They comprise one element fixed to the shaft and a second element on the housing or body. These are arranged with surfaces that are perpendicular to the shaft axis and are brought together so the rotating shaft element rubs against the fixed housing element.

This perpendicular configuration accommodates runout between shaft and housing. A small gap – less than 0.0001″ – between the two contacting surfaces stops particles getting into the fluid but can allow a degree of leakage. The alternative is to put combinations of low friction materials like graphite and ceramic or carbide in contact. This drives leakage rates almost to zero. For additional sealing, many modern mechanical seals spring-load one element to push it against the other.

Contact Hennig

Hennig Gasket & Seals are experts in all types of gasket and sealing applications. Contact us for gasket materials or mechanical seal rebuilds.

Soft Rubber Gasket Material

Uneven sealing surfaces need a soft rubber gasket material that deforms to fill the space. Covers and enclosure doors are examples where the closed gap often varies. A soft material will seal better than one that’s firmer.

Gasket material softness is measured by durometer and expressed in terms of Shore A and D. Understanding this will help when it’s time to order soft rubber gasket material.

Shore Hardness Scales and Durometer

Rubber and plastic firmness is measured by pushing a hard point into the material and measuring the indentation. The measurement system used is the Shore hardness scale, named after its inventor, Albert Shore.

Shore’s indentation device is called a durometer. This is why rubber hardness is sometimes spoken of in terms of durometer, or “duro.”

As the hardness or rubbers and plastics varies widely, Shore defined several scales but for most practical purposes Shore A and Shore D are enough.

Shore A and D Values

Shore A covers softer materials, Shore D those that are firmer. Both scales run from 0 to 100. They overlap, so rather than specify the top end of the A scale it’s more usual to use the middle of the D scale.

Here are typical Shore values for common materials:

  • Rubber band – 20 Shore A
  • Pencil eraser – 55 Shore A
  • Shoe soles – 70 Shore A
  • Leather belt – 80 Shore A.
  • Golf ball – 50 Shore D
  • Shopping cart wheels – 60 Shore D.

Shore Values and Soft Rubber Gasket Material

The softest material generally used for gasket applications is 30 duro, or Shore A, neoprene. Slightly firmer neoprene, nitrile and EPDM are all available in 40 duro / Shore A.

Foam is usually softer than solid material, as the air pockets provide additional compressability. Open cell foam is softer than closed cell in the same material, but will let fluids through.

Advice on Soft Rubber Gasket Material

A soft, closed cell rubber material is better for sealing uneven surfaces like those on doors and covers. The firmness of these is usually specified in Shore A. Ask us for help in choosing the best material for your application.

Food Grade Gasket Materials – the Basics to Know

To avoid contamination, food and beverage handling and processing equipment is generally fabricated from materials like stainless steel. Gaskets can’t be avoided though, and some will come into contact with the products being manufactured. When this is the case it’s essential they are made from gasket material considered food grade.

The FDA publishes a long list of substances that can safely be included in elastomeric (rubber-like) gaskets: it’s in 21 CFR 177.2600. However, relating that to common gasket materials is no easy task. Asking a gasket material supplier for their expert advice and recommendations is a better approach. Here’s what you should know.

Food-Grade Material Choices

Commonly used gasket materials are also available in grades considered food-safe. That means material choice is dictated by what the application needs. Only once temperature, environment, media and pressure, (summarized by the acronym TEMP,) are known do you need to add the “food grade” requirement.

Silicone is often the first choice when food grade is required. It’s an excellent gasket material with a wide temperature range, low compression set, good strength and elasticity and resistance to many chemicals. In some applications though it may be overkill. Nitrile, neoprene, EPDM, Gylon®, Viton® and PTFE are alternatives that may perform better while possibly saving money.

Not Necessarily White

An absence of color-adding substances mean food-grade gasket materials are usually white. However, there are alternatives. Food-grade EPDM for example is available in black, and food-grade silicone may be translucent or other colors.

The bottom line is, don’t assume your food-grade gasket material must be white. If you need something else, ask us.

Contact is the Important Point

Consider food-grade material mandatory for gaskets coming into contact with food. Elsewhere in a food or beverage facility though, you can use other materials. To avoid confusion, a plant may decide to use only food-grade material everywhere, or implement appropriate controls to prevent misuse.

Seek Specialist Advice

If you’re responsible for food handling and processing equipment you need to know about food grade gasket materials. You could spend a lot of time on research, or you could come to us instead.

PTFE Gasket Sheet – Three Things to Know

PTFE is an extremely useful gasket material. So useful that it’s sometimes purchased in sheet form for cutting to shape as needed. Here we’ll talk about the strengths of PTFE gasket sheet and then we’ll discuss three things you should know.

Pros for PTFE Gasket Sheet

PTFE is chemically inert, which makes it a good material for sealing against almost everything except fluorines and nitric acid. It also has a very wide temperature range. Grades vary but it’s generally usable from -100 to 500 °F. Famously slippery, its low friction surface is advantageous in some applications.

Your gasket supplier can cut PTFE gaskets to shape (waterjet and die cutting both work well,) or you can buy sheet PTFE and cut gaskets yourself. Thin sheet is readily cut by knife and thicker sheet is easily milled.

Important Things to Know

Besides the strengths of this material, it’s important to be aware of:

  • Tendency to creep
  • Limitations for food applications
  • Sheet thickness tolerances


Creep is a progressive, non-reversible deformation that occurs under load. This reduces clamping force, which eventually causes leaks.

The solutions are to either use it only in low-load applications, which is often impractical, or use a filled grade. PTFE fillers include glass, carbon, graphite, bronze and other materials, sometimes in combination. These raise strength but at the expense of a less slippery surface and reduced chemical resistance.

Food Limitations of PTFE Sheet Material

Pure PTFE is so inert it can be used in applications where it comes into direct contact with food. This is not true for the filled grades. These are likely to discolor or taint foodstuffs so are not permitted in such applications.

Sheet Thickness Tolerances

Thickness is an important gasket characteristic. Thickness tolerance depends on how the PTFE sheet is produced. The options are molding or skiving. Skiving entails cutting a thin layer away from the surface of a cylinder of PTFE, and it produces a more consistent sheet. For precise PTFE sheet thickness, specify skived material.

PTFE Sheet From Hennig Gasket & Seals

We can supply molded and skived sheet 60” square. Thicknesses range from 1/32” to 4”. Contact us to learn more

Hi-Temp Gasket Material Applications

Most gasket applications are satisfied by various forms of synthetic rubber. Neoprene, NBR and SBR for example all work well when temperatures are below 200 °F. Above this, things get more complicated. If you have an application for hi-temp gasket material, here’s what you should know.

How Hot is Too Hot?

Gasket material specifications sometimes give a range for the upper temperature limit. This is because there’s no single temperature at which a material stops working. Instead, it breaks down gradually.

Heat hardens rubber and rubber-like materials, but time at temperature is as important as the peak value the material sees. Pressure is another factor: in a high-pressure application it might be prudent to use a material with a higher upper temperature limit.

Most commonly-used gasket materials top out at around 200 – 225 °F. If you have an application where temperatures are consistently above 225 °F consider exploring options for high temperature gasket material.

Hi-Temp Gakset Material:  Elastomeric Materials

With a sustained temperature limit of 225 °F, EPDM is an elastomer that goes hotter than NBR, SBR and Neoprene. Most grades of silicone, fluorosilicone and fluorocarbon (Viton) gasket material will endure 500 °F, and 600 °F for short periods.

Hi-Temp Gasket Material:  Compressed Non-Asbestos and PTFE

Compressed non-asbestos materials are composed of fibers, (mostly aramid,) in an elastomer binder such as NBR or SBR. The fibers provide strength and pressure resistance and the binder holds them in place.

Compressed non-asbestos materials are made in many grades and formulations but most will withstand up to 750 °F. In contrast, the limit for PTFE is 600 °F.

Very High Temperature Gasket Materials

Choices are limited for temperatures above 750 °F but one of the most widely used materials is vermiculite (a clay-like mineral.) This is either applied to a solid gasket or incorporated into a spiral wound gasket for pipe joints. Its upper temperature limit is around 1,800 °F.

Advice on Gasket Materials for High Temperatures

High temperatures are bad for many gasket materials. As gasket material suppliers of a large inventory of materials, ask us for a material quote if you need to seal hot fluids or in a hot environment.

Fiber Gasket Material – It’s Broad Meaning

Although the word “fiber” is often used to describe sheet gasket material, fiber gasket material is a very broad category. In this blog post we’ll explore what types of material count as “fiber” and when and why you might want to use them.  Hennig Gasket & Seals is gasket material suppliers of all types. 

Two Main Categories of Fiber Material

Fiber” can refer to vegetable fiber gasket material, or to sheets composed of fibers bound in an elastomeric matrix. Each provides varying levels of the temperature and chemical resistance, strength, and conformability needed in a good gasket, but there are differences between them. What’s more, within each category there are many variations of composition.

In addition, “fiber” can also mean ceramic fiber or cellulose-based gasket material. Those are quite specialized and won’t be covered here, but if you’re interested in them the specialists at Hennig Gasket  can help.

Vegetable Fiber Gasket Material

This material is made by a similar process to papermaking. Plant material is crushed and pulped to leave a mass of fibers. These are spread and dried as sheets.

Vegetable fiber gasket material, often referred to as “Detroiter” material, is very thin and has good dielectric properties. It’s resistant to water, air and most fuel oils, as well as gasoline and benzine. Adding cork to the mix results in a thicker, more compressible material.

Compressed Fiber Gasket Material

This is made by mixing aramid fibers in an elastomer like NBR or SBR, then rolling it into sheet form. Aramid is a contraction of aromatic polyamide, which is a high-strength synthetic polymer. (One of the tradenames it goes by is Kevlar.)

In gasket applications aramid supplies strength and temperature resistance while the elastomeric binder provides compressibility. Manufacturers tailor the material to specific gasket applications through elastomer selection and by varying the amount of aramid used.

Compressed fiber gasket material offers strength, good compressibility, and creep-resistance. Chemical resistance depends on the type of elastomer incorporated. As gasket material suppliers of a wide range of compressed fiber gasket material from well-known manufacturers like Garlock, Klinger, UTEX, Flexitallic and JM Clipper, we have the material to accmodate your application needs.

What is Vellumoid and When Would You Use It?

The name may be unfamiliar but Vellumoid is one of the oldest gasket materials. Age hasn’t rendered it obsolete though and there are still applications where it may be a good choice. In this blog post we’ll explain what Vellumoid Gasket Material is and where you might want to use it.

A Mix of Paper, Glue and Sugar

Vellumoid is cellulose fiber impregnated with a protein glue and gylcerin. It was developed back in the 1890s. Apparently seeking to develop a waterproof paper, the inventor created something that looked and felt like vellum, (a high-quality form of parchment.) And from those humble origins, they named it “Vellumoid”.

Today the Vellumoid company, based in Worcester, MA, has grown an entire family of materials from this simple beginning. The original material is sold as “Branded Vellumoid” while variants are impregnated with other materials that perform well in gasket applications.

Properties and Performance of Vellumoid

The original Vellumoid is very water-resistant, which was the original goal, and also holds up well against antifreeze, water-based cutting fluids, gasoline and lubricating oils. It’s also mold-resistant, (by virtue of the glycerin,) so is often a good choice in damp locations.

Compressibility is excellent, which is one of the traits that make it interesting for gasket use. However, tensile strength is low and the upper-temperature limit is 250⁰ F (121⁰ C).

Other variants of Vellumiod incorporate materials that improve properties. Velbuna WG-5 for example is impregnated with nitrile and withstands up to 300⁰ F (149⁰ C) and Vellutherm 600 goes up to 350⁰ F (177⁰ C).

When to Use Vellumoid

Excellent compressibility makes it good for sealing uneven flange surfaces. That’s something you might encounter when rebuilding or restoring vintage machinery, and where Vellumoid might help add authenticity. Of course, it will work just as well in modern piping systems carrying hot or cold water or similar fluids.

If you think Vellumoid might be a good choice for your application, contact us. We sell vellumoid in sheet form and can cut it to the size and shape you need.

SBR Gasket Material: Don’t Overthink It

You have a lot of options when you’re looking for gasket material, and some of them can get very expensive. While it may be tempting to select a highly engineered polymer with exceptional temperature and chemical resistance properties, ask whether that’s what you really need. In many cases, you may be better off using Styrene Butadiene Rubber, better known as SBR Gasket Material.

SBR Gasket Material:  An Alternative to Natural Rubber Gasket Material

SBR red rubber gasket material was developed during WWII. At the time most rubber came from Asia, and as more rubber was needed for tires, scientists were searching for an alternative. Styrene-Butadiene Rubber, derived from the Buna polymer developed in Germany in 1929, met that need.

Natural rubber has many things in its favor as a material for gaskets as well as tires. It’s soft and elastic, with good compression recovery. That lets it conform to uneven flange faces and expand to fill the gaps. However, it doesn’t fair well outdoors and its temperature range and chemical resistance are so-so at best. SBR improves on natural rubber with a wider temperature range and better weathering and chemical resistance properties. It’s also a very economical choice.

Don’t Over-Engineer the Application

SBR Gasket Material isn’t the highest performing gasket material available, but for many applications, it’s good enough. Use it when temperatures won’t go above 170° F or below -30° F and for sealing against air, water and steam. Use it when flange faces may be uneven, and use it in applications that need good impact strength and abrasion resistance.

Don’t use SBR gasket material to seal against hydraulic fluid, solvents or oils as this will make it swell excessively. Neither is it a good choice for outdoor applications with exposure to UV and/or ozone.

Many alternative gasket materials provide higher performance but at a higher price. Rather than considering what might happen in-service, focus on what’s known for sure. You may find that SBR is up to the task and will save you some money. For more information, discuss your application with the specialists at Hennig Gasket & Seals.

Gum Rubber Sheet: Cushioning and More

Gum rubber sheet is natural rubber in an industrial form. It’s very flexible, has great elongation, high resilience, and resists abrasion and tearing. This makes it one of the best materials for cushioning against impacts and damping vibration. There are a lot of man-made, synthetic versions of rubber but for many applications, and especially for cushioning, it’s hard to beat the original, natural gum rubber sheet material.

Gum Rubber: An Elastomer That’s Grown

Gum or natural gum rubber is made from the sap of the rubber tree, Hevea brasiliensis. The tree is cut to release the sap, also known as latex. This is allowed to coagulate before being passed through rollers to produce sheets. From here additives go in and the rubber is vulcanized to increase cross-linking. The resulting material is chemically very similar to synthetic rubbers like SBR and nitrile.

Properties of Gum Rubber Sheet

As a natural product, there’s always some variation in how gum rubber behaves. In general, though, tensile strength is around 2,800 psi, elongation is a hard-to-beat 550%, and compression set is very low. Gum rubber is known for being extremely flexible, and it remains so down to around -40 °F. Its upper-temperature limit is a relatively low 140 °F, making this one of its limitations.

Gum rubber resists attack by most acids, alkalies, and organic salts, (such as citrates.) Its chemical weaknesses are oils, gasoline, and hydrocarbon solvents. It’s also susceptible to attack by UV light and ozone, which means it’s not a material for prolonged outdoor use.

Gum Rubber Sheet Applications

Flexibility and resilience make impact absorption one of the main uses of this material. That’s why it’s a good choice for cushioning pads and bumpers and why it’s also used for vibration absorption.

Gum rubber is used in seals of various types, such as around doors and for skirting around containers and hoppers because it’s very flexible. Conveyor belt scrapers are another use.

Ask Hennig Gasket & Seals for Gum Rubber Sheet available up to 48″ wide and a range of thicknesses.