8. Parbak® back-up rings

Introduction

The Parker Parbak is a supplement to pressure seals such as an O-ring. It is not a sealing ring in itself. The primary reason for the use of a back-up ring is to reduce the clearance gap on the low pressure side of an O-ring or similar type of seal. An O-ring and Parbak combination will withstand much higher pressures than an O-ring alone. Parker Seal´s Parbak® back-up rings are outstanding for several reasons. To begin with, Parbaks® are moulded in a hard rubber compound in one continuous circle, yet they can be stretched easily into place. Because they are not split or spiralled, Parbaks® do not cause the localized wear on O-rings which is characteristic of other types of back-up rings.

Parbaks® have proved to last longer with greater reliability than other types of back-up rings tested. Parbaks® extend the op- erating pressure range of O-rings or other types of seals. The additional benefit of increased lubrication of the O-ring may be also obtained because the Parbak® will trap small amounts of lubricant between itself and the O-ring.

The use of Parbaks® permits a far greater metal-to-metal clear- ance between moving parts. This helps overcome difficulties encountered with wide or out-of-tolerance bores and shafts.

The elastic nature of O-ring seals and the clearance reducing capability of the Parbak® offers an important cost reduction op- portunity. In many applications, machining tolerances can be increased at considerable savings in time and money

• Easy to assemble – Parbaks® are engineered to exacting requirements and manufactured to close tolerances. Their stretchability makes them easy to assemble, requiring much less time than other types of back-ups. They cannot fall out after being put into place. Because they work equally well with the contoured side facing each way, there is no problem with Parbaks® being installed backwards.
• Cost reduction item – O-rings will seal effectively within certain clearance gap limitations (see Fig 8.5). The use of back-up rings extends the clearance gap limitations allowing looser fittings of moving parts. This makes up for production tolerances and saves time and money in machining require- ments and bulky designs. Because Parbaks® permit larger clearances and larger tolerances, lighter and more flexible cylinder walls can be allowed.
• Shaped for better performance – Parbaks® contoured design, whichever way it is installed, provides improved performance. The shape tends to keep the O-ring round even under high pressure. With proper groove designs, Parbaks® will not “col- lapse” and never cold flow into the clearance gap.
• Low price – Parbaks® are low-priced in comparison with other types of back-ups.
• Longer O-ring lives – Continuous construction means Par- baks® do not have break-apart cuts, spirals, or other discon- tinuities which can allow O-ring extrusion, pinching or biting. They minimise the chance of misalignment with the resultant cutting and tearing action on O-rings. They therefore are more reliable and greatly extend O-ring lifetimes.
• Improved lubrication – Due to their design, compound and configuration, Parbaks® help provide adequate lubrication of O-rings. Some back-up rings (such as PTFE types) deposit a film on the metal surfaces preventing adherence of lubricants which results in more rapid wear of O-rings.
• Resistance to higher pressures – Parbaks® are moulded in special rubber materials designed specially to resist higher pressures, last longer and improve seal performance.
• Uniform materials and sizes – Through rigid quality control in every stage of manufacturing and inspection, Parbaks® are consistent in compound and dimensions. They are readily available in standard sizes through a nation-wide network of distributors. Special sizes can be obtained for nominal extra preparation charges.

Design information

Hardness must be considered when choosing an O-ring. For most seal applications optimum performance is achieved with a 70 Shore A durometer compound. Generally, O-rings in this hardness range seal well at low pressures. However, they are more liable to extrusion at high pressure than harder rubber parts. O-ring seals which exceed a Shore A value of 85 durometer are seldom successful in dynamic applications because the harder materials  do  not  follow  irregularities or deformations in the sealing surface (e.g. cylinder wall). Hence, harder O-rings tend to permit leakage, particularly at low pressures. Using Parbaks® is a simple way to obtain the low pressure sealing ability, wear and abrasion resistance of the lower durometer O-rings plus the high pressure extrusion resistance of a hard compound. Standard Parbak® com- pound N 300-90 hardens further at elevated temperatures enhancing its extrusion resistant quality.

• The Parbak® order numbers correspond with Parker O-ring 2-xxx series and can be assembled together in a groove without difficulty. The order number for a Parbak® ring cor- responding to an O-ring 2-211 standard compound N 300-90 is 8-211, N 300-90.
• The groove width b1 or b2 can be taken from Table 8.1, design details are found in Section 2.

The surface finish for dynamic applications should be taken from Table 8.2. In general it can be said that the working lifetimes of both O-rings and Parbaks® are extended by im- provement of the surface finish. The surface finish roughness should in any case not be less than Rmax = 0.5 mm because a lubricating film needs to be encouraged. Rapid wear is the result of an insufficient lubricating film.

• Whenever possible, two Parbaks® should be provided to reduce the risk of incorrect fitting (i.e. Parbak® at the wrong side of O-ring).
• When high pressure is only from one side, the Parbak® must be positioned correctly. Keep in mind: pressure – O-ring
• Parbak (with its concave surface facing the O-ring).
• The O-ring groove bottom radius must be as small as pos- sible taking into account the strength of the materials and the difficulty of machining.

The O-ring and the Parbak® must be selected to suit the ap- plication and considering all factors (e.g. pressure, tempera- ture, contact medium, velocity, size, etc.).

Allowable range of pressure

Fig 8.6 shows the extrusion characteristics for elastomers of dif- ferent hardnesses. When considering the risk of extrusion at a particular pressure and diameter clearance gap, the tendency  of metal parts to “breath” under fluctuating pressures cannot be ignored. The total play in diameter must be taken because in general eccentricity is to be expected within limits.

Example: O-ring compound: N 674-70 (70 °Shore A)

Working pressure: 100 bar

Play in diameter: 0.36 mm max to drawing tolerances and 0.40 mm when “breathing”

An O-ring of 70 Shore hardness would tend to be forced into    a diameter gap of 0.4mm by a pressure of appr 38bar. When Parbak® back-up rings (nominal hardness 90 Shore A) are used, in addition to the O-ring, a maximum pressure of 140 bar is al- lowable. In addition to pressure and temperature, other factors play an important role in interpreting the extrusion diagram. To allow for these influences (e.g. of temperature and wear), a safety factor is necessary. 

Compounds

Parbaks® are available as standard in compound N 300-90, an elastomer based on acryl-nitrile butadiene rubber (NBR) with a hardness of 90 Shore A. Its physical properties make it suitable for most applications: wide temperature range, correct hardness, long life, compatibility with a wide range of media including near- ly all hydraulic fluids but not including fire-resistant fluids such as Skydrol, Pydraul, Houghtosafe, Hydrodrive, etc. N 300-90 also is compatible with air and water. The working temperature range is between -55°C and +105°C.

For particular applications, Parker has developed other Parbak materials in  compounds  possessing  special  properties,  e.g.  E 625-90 (ethylene-propylene) or V 709-90 (fluorocarbon). Parbak® standard sizes (8-xxx series) with tolerances for A, M, R, T and W (Fig 8.7) can be taken from the table “Back-up ring standard sizes”.