Wednesday, May 18, 2016

Rotary Seal Lip Design and Geometry

Probably the most critical design feature of any seal design is the contact point or interface between the sealing lip and application shaft or housing if it’s an external seal. Typically, the sealing lips are made of an elastomeric material such as NBR, Viton, PTFE, etc.

Apart from the elastomeric lip material, a critical design feature of the seal lip is its geometry. Lip geometry is one of several factors that directly affect the radial load exerted on the shaft. The others include the
presence of a garter spring, shaft finish, pressure and installation. Since the inner diameter of a seal is manufactured to be slightly smaller than the shaft diameter, the elastomeric lip will be stretched outward by the shaft. When a garter spring is added to the seal, these two forces contribute to increase the load on the shaft. Two lip measurements that contribute to the radial load force include the beam or lip length measured as the parallel distance between the point where the lip contacts the shaft and thinnest part of the beam. Another factor contributing to the radial load on the shaft is the beam or flex thickness. Flex thickness – the width of the area between the sealing lip and heel of a shaft seal. This width should be thick enough to
prevent the lip curling back on itself but thin enough to allow follow-ability. This measurement has direct influence on the radial load force or pressure on the shaft. For instance, for any fixed beam thickness a longer lip length will exert less load force on the shaft than a shorter lip length. Conversely, for the same beam thickness a shorter lip length will exert more force on the shaft. Given the above criteria, a shorter lip will also experience increased friction and wear than a longer lip. It also follows that for a fixed lip length any
increase in flex thickness will result in increased radial load force on the shaft whereas decreasing the flex thickness will reduce the radial load force. In general, the beam should be thick enough to prevent torsional distortion and thin enough to allow flexibility. In high-pressure applications, shorter lips are preferred because they have higher resistance to deformation. For shafts with eccentricities such as shaft to bore misalignment or dynamic run out, longer lips are preferred because they are more flexible resulting in superior follow-ability. Follow-ability is the ability of the sealing lip to maintain contact with the shaft under dynamic run-out conditions. The ideal lip can only be determined by evaluating several interrelated factors including shaft speed, presence of eccentricities, lip materials etc. Care must be taken when increasing the lip length due
to the possibility of it twisting on the shaft. Application engineers should consult their seal manufacturer, if unsure of what style of lip to use.

Lip Seal Glossary
1. Case Width
2. Outer Case
3. Housing
4. Inner Case
5. Outside Face
6. Lip Length
7. Inside Face
8. Radial Wall Dimension
9. Seal Outer Diameter
10. Housing Bore
11. Spring Axial Position
12. Spring Groove
13. Garter Spring
14. Axial Clearance
15. Heel Section
16. Flex Thickness Area
17. Spring Retaining Lip
18. Head Section
19. Inside Lip Angle
20. Toe Face
21. Inside Face Inner Diameter
22. Outer Case Inner Diameter
23. Secondary Lip
24. Outside Lip Surface
25. Airside Lip Angle
26. Rib (Hydrodynamic Seal Only)
27. Contact Line
28. Static Lip
29. Fluidside Lip Surface
30. Molded Toe Angle
31. Lip Diameter
32. Unsprung Lip Diameter
33. Contact Line Height
34. Lip Height

Another key design feature in the design of a seal lip is the angle on both sides of the interface where the seal lip and shaft contact. In general, it is recommended that seal lips be designed with a fluid side angle that is greater than the airside angle. Fluid side angle - angle between the side of the sealing lip facing the fluid being sealed, and the shaft. Airside angle – angle between the side of the sealing lip facing away from the fluid being sealed, and the shaft. If a seal is installed backwards, the airside lip angle will be greater than the fluid side angle creating a situation where more fluid is pumped towards the contact zone resulting in leakage. When influenced by shaft finish, fluid viscosity, temperature and radial load, the fluid side angle creates a pressure gradient that is greater than the air side pressure gradient. This helps create a condition where more fluid is pumped away from the contact zone than towards it. Under RMA standards, the shaft is finished to not leave any screw grooves from the machining process. This is called plunge grinding. Shaft finish is important because as the shaft rotates it will wear away a little rubber from the seal tip. If the shaft is to smooth, then not enough lip abrasion will occur. If the shaft is too rough, then excessive wear will occur. As optimal wear occurs, mircoaspreties will form on the seal lip. Microaspreties – microscopic pores that develop on an elastomeric sealing lip as a result of wear caused by a rotating shaft. Microaspreties hold lubrication and facilitate a pumping action that prevents leakage. As the shaft rotates, the microasperities are stretched in a circular direction at an angle to the shaft, creating a mini helix. When influenced by the lip angle, shaft speed, fluid viscosity and other operational characteristics these miniature helixes create an inward “pumping” action that pushes fluid back into the reservoir. Helixes can also be molded onto the sealing lip during the manufacturing process creating a hydrodynamic seal. Hydrodynamic seal –s a specially designed seal that features helical ribs, cross etched designs or grooves molded onto the airside of an elastomeric sealing lip. These helicies force fluid back into the reservoir. Typically used in uni-directional shaft rotations. The helix designs are typically ribbed, pads or other cross-etched designs. Because of the vacuum crated by high pump rates, helixical designs also push contaminants towards the seal lip which may increase lip and shaft wear. In addition, helixical designs should only be used on shafts rotating in one direction. Reversing the shaft rotation will cause leakage because the pumping action is reversed causing fluid to be pumped out of the sump. In addition, installing a seal backwards will also cause significant leakage because the pumping action will now be away from the reservoir. To facilitate a bidirectional shaft, it is recommended that triangular pads be molded onto the lip. Finally, some seal designs also include the use of a non spring-loaded secondary sealing lip. The secondary sealing lip is typically used to exclude airside contaminants.

Secondary lips are generally smaller in size but have a larger diameter than the primary lip and are located in the heel of the beam (the opposite of the primary lip head). Secondary sealing lips can have an axial or radial orientation to the shaft. Axially oriented secondary sealing lips will require a wall or flange to seal against and are used in applications that require superior dirt exclusion. Radial oriented secondary sealing lips seal against the shaft and generally exclude light dust and contaminants. Seal lip design is influenced by a numerous operational and design factors. Application designers should consult their seal supplier for seal design recommendations for their applications. If you have any questions or require further information on what type of seal to use for your application, contact Colonial Seal Company at 1.800.564.2201 or email 

Founded in 1994, Colonial Seal Co. is headquartered in Westville, New Jersey. Colonial Seal specializes in oil & grease seals, hydraulic seals, rotary shaft seals, mechanical seals, rubber molded products and gaskets. Through a global network of manufacturers, Colonial Seal can produce custom seals designed for a client’s unique sealing applications. Colonial Seal Company is ISO 9001:2008 certified.

Tuesday, May 10, 2016

Overview of PTFE fillers used in Rotary Seals, Wear Rings, Piston Rings and Related Sealing Products

PTFE (Polytetrafluoroethylene) has become a commonly used material in sealing applications where other materials do not meet temperature, chemical compatibility, friction, or wear requirements. Many customers are unaware that “fillers” may be added to PTFE in order to enhance certain characteristics not found in virgin PTFE. Virgin PTFE possesses “creep” behavior that reduces effectiveness in mechanical applications.
Significant deformation of virgin PTFE may occur over time even at room temperatures. We recently had a customer that indicated that the ID (Inside Dimension) of their virgin PTFE seals was significantly less than the requirement. This seals had been in inventory for 6+ months and deformed over time without anything used to retain the original dimension/shape. Virgin PTFE also wears quickly. The use of fillers improves the physical attributes of virgin PTFE, including creep and wear resistance. Which filler to use certainly depends on the application. Some of the common fillers we have used to meet customer requirements include:

•Glass is the most common filler for PTFE. Widely used in hydraulic piston rings, glass gives good wear resistance, low creep, and good compressive strength. Glass also has excellent chemical compatibility. The percentage of glass varies between 5 and 40%. The major disadvantage is that glass-filled PTFE compounds are abrasive to mating surfaces, especially in rotary applications.

•Molybdenum disulfide (MoS2) improves wear resistance and further lowers the coefficient of friction. “Moly" is typically combined with other fillers (such as glass and bronze).

•Carbon imparts excellent compression (low deformation under load) and wear resistance, good thermal conductivity, and low permeability. Carbon-filled PTFE compounds are not as abrasive as glass-filled compounds, but they are still more abrasive than polymer-filled compounds. The percentage of carbon added varies between 5 and 15%. Carbon-filled compounds have excellent wear and friction properties when combined with graphite. Carbon fiber lends better creep resistance than carbon powder, but fiber is more expensive.

•Graphite is a crystal modification of high purity carbon. Its flaky structure imparts excellent lubricity and decreased wear. Percentages of this filler vary between 5 and 15%. Graphite is often combined with other fillers (especially carbon and glass).

•Bronze lends excellent wear resistance and thermal conductivity. Bronze-filled materials have higher friction than other filled PTFE compounds, but that can be improved by adding moly or graphite. Bearing and piston ring applications often use compounds containing 55% bronze – 5% moly. Bronze-filled compounds have poorer chemical resistance than other PTFE compounds. Bronze, when used as a filler, is added in percentages of weight between 40 and 60%.

•Ekonol® is thermally stable aromatic polyester. When blended with PTFE, it produces a composite material with excellent high temperature and wear resistance. Ekonol® will not wear mating metal surfaces, making it good for rotary applications. Ekonol®-filled materials are also good for food service.

•Polyimide is another type of polymeric filler offering superior wear and abrasion resistance. Polyimide-filled PTFE compounds have about the lowest friction properties of all filled PTFE materials, so they provide great performance in non-lubricated (dry) applications. They will not abrade mating surfaces (even soft materials such as brass, stainless steel, aluminum, and plastic). Polyimide is one of the most expensive PTFE fillers, however. Determining the percentages or weights of fillers is not an exact science, but there are mechanical and thermal test results available for many of these PTFE fillers. Chemical compatibility charts are also available to assist in selecting the proper filler for a given application. “Filled” PTFE products that have been produced for our customers include: bronze-filled PTFE rings for a master cylinder application; graphite filled PTFE in a pump sealing application; carbon filled PTFE rotary seal for a bearing company application; 15% glass and 5% Moly for a pharma customer in a rotary application; a 40% bronze filled PTFE wear ring for an FDA compliant application; 10% Ekonol PTFE for a spring energized seal; 25% carbon filled guide ring for a mixer MRO; and a 25% glass filled PTFE FDA compliant for a pharma agitator manufacturer.

Founded in 1994, Colonial Seal Co. is headquartered in Westville, New Jersey. Colonial Seal specializes in oil & grease seals, hydraulic seals, rotary shaft seals, mechanical seals, rubber molded products and gaskets. Through a global network of manufacturers, Colonial Seal can produce custom seals designed for a client’s unique sealing applications. Colonial Seal Company is ISO 9001:2008 certified.

Thursday, May 5, 2016

Burr Identification and Removal

If you’re in the business of milling, grinding, drilling, plasma cutting, stamping, or any of the other various processes used to produce machined components, then you are all too familiar with burrs and the problems they cause. Burrs are a nuisance that draws close comparisons to the common cold. Just as there are many
different ailments associated when diagnosing an individual with your “typical cold,” there are multiple definitions that attempt to describe what constitutes a burr and the root cause of these deformations. One generally accepted description, provided by the Society of Manufacturing Engineers (SME), defines a “burr” as a raised edge or small piece of material that remains attached to a work piece after a modification process. For the purpose of this article, the primary focus will be centered on identifying burrs created by the molding and machining processes during the manufacturing of various types of metal-cased seals, and how these burrs can be properly removed.

Just as any machine part experiences wear from prolonged use, the seal molds (tools), which are
used to caste the various shapes and styles of seals, also experience the same degradation after a certain
working life period. As the tool is exposed to this prolonged use, the edges of the tool itself will slowly begin to deteriorate and lose their structural rigidity. When the mold’s edges begin to wear away, excess metal alloy may escape through tiny gaps resulting in a seal casing that has excess metal deposits, or burrs. These excess deposits can become a problem if not removed prior to being installed in a particular application. Burrs increase the risk of corrosion, could cause unwanted friction, reduce the sealing between the seal case and bore, and in rare instances act as an electrical conductor.

Because the excess metal does not allow for the seal to fit properly into the application, one side of the seal may be raised higher than the other side. This allows air or moisture to directly contact the metal seal casing, which is one of the primary causes of corrosion and could lead to premature seal failure or make it very difficult to remove the seal when replacement is required.

When the seal is being installed in a dynamic application, burrs could cause unwanted friction, improper lubrication, or damage to the shaft or bore. If the burr is located on the inner diameter of the seal, it could create scratches or wear marks on the shaft, thereby causing excessive, undesirable leakage. Conversely, if the burr is located on the outer casing of the seal, it could cause damage to the housing, or bore. In the most extreme instances, if a seal with a burr is installed in a dynamic application, the sharp point of the burr can act as an electrical conductor, which can result in a static discharge, and depending on the media used in the application, the possibility of fire or explosion may exist.

Unfortunately, identifying burrs on a seal casing is not usually as simple as a visual inspection. This is the reason some burrs are not detected until the seal is already installed in an application, and the burr results in a seal malfunction or causes unnecessary wear or damage to the shaft or housing. Most seal manufacturing processes incorporate a finished goods inspection step in their Quality Management System. For burr detection this step would include having a technician or quality inspector test a representative sample amount from each batch of product prior to final approval of the product. This quality control step will certainly help
in detecting these deformations and should provide the manufacturer an indicator of tool wear. Proper
testing procedures for burr detection include both a visual inspection using a magnification tool or a physical “touching” process with the hand of the inspector. The limits on size or number of burrs is usually set by the manufacturer, but can also be specified by the customer. If burrs are detected and are not within tolerances, there are several methods of “de-burring” or removing these imperfections so that the seal cases still meet customer requirements. Of course, the tools should also be re-inspected to ensure that the tool meets design specifications.

The process of removing these extrusions or burrs from what should be smooth, finished surfaces is referred to as “de-burring.” De-burring can be completed either manually or automatically. Manual de-burring has been around for decades and involves a worker physically searching, finding, and removing burrs using a manual de-burring tool (most often a type of brush with steel, nylon, or diamond-coated bristles). Those who are involved in this activity are noted for their patience, dexterity, and attention to detail. However, this process creates a large amount of metal dust, which can be hazardous to one’s health, and the process is extremely repetitious, which often results in inconsistent dimensional “finished” products. Most manufacturers have automated the de-burring process. This ensures that each finished product is subject to the exact same procedures for product de-burring, as opposed to a manual process that sometimes produces inconsistent results.

Automated de-burring can be completed using a variety of different techniques including brush-ing, milling, grinding, and tumbling. For brush deburring, the de-burring tool rotates on an axis and the brush material conforms to the product’s surface removing any excess extrusions. This technique of burr removal relies heavily on the rotating speed of the brush, as well as the bristle flexibility. Factors that affect flexibility include the bristle material, length, diameter, and tensile strength. Typical cutting speeds for these de-burring applications range from 15 meters per second to 30 meters per second. The milling and grinding de-burring processes are required when burrs have a foot width (distance the burr extends along the product surface) in excess of 0.4 millimeters. Milling and grinding require guiding the tool precisely along the product’s edge that is being de-burred. Pressure applied to the tool against the edge that is being de-burred (known as the “expansion pressure”) determines how much excess metal will be removed from the case or metal product. Unfortunately, milling and grinding create secondary burrs that must then be removed using the brushing technique. The third option for deburring is referred to as “tumbling.” It is the most inexpensive option and allows for multiple products to be de-burred at the same time. This process involves placing the products with the deformations into a rotating barrel or vibratory bowl along with some type of finishing media (when dealing with metal alloys, the most popular finishing product is a type of ceramic media). This option for de-burring is best suited for unfinished materials. The reason for this is because the media that removes the burrs will also remove whatever type of finishing is on the product whether it is a polished plate or some type of paint. For finished products, one of the previously described de-burring methods should be utilized, which will increase lead times and costs.

In summation, burr identification and removal processes are both issues that in an ideal world would not exist. Fortunately, most manufacturers have implemented appropriate identification techniques and removal procedures that minimize the occurrence of burrs in finished products. Whether you rely on an automated process like tumbling or on a worker in the warehouse, it is imperative that programs are in place for monitoring products and properly removing burrs from any unit prior to it reaching the end-user. Distributors should also have random inspection procedures in place for finished product that includes burr identification, especially if customers’ print specifically requires that no burrs are acceptable.