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As an auxiliary seal of mechanical seal, not only sealing performance but also friction force that’s generated during use need to be considered in O ring seal design.For friction forth plays a vital role in how mechanical seal work. Most of design parameters of O ring seal are related to friction forth.
Friction forth of O-ring seal includes adhesive friction and hysteresis friction forth(s). Hysteresis friction force is smaller than adhesive friction force, so when estimating the influence of friction force of O-ring seal on the rotary face and stationary seat, it is safer to use adhesive friction force for calculation.
Adhesive friction forth is composed of two parts, and the calculation formula is as follows:
Fa = Fc L + FH Ap
Fa is adhesive friction force, measuring unit is N；Fc is compression friction coefficient of the O-ring seal, N/m；FH is pressure friction coefficient of the O-ring seal, MPa； L is the contact length of the O-ring seal, m ;Ap is the projected area of the O-ring seal after assembly, m2
Among them,Fc and FH have little relevance to type of material, but are related to hardness and compression rate of the material, and can only be measured through experiments.The greater the hardness of the material, the lower the friction; the lower the compression rate, the lower the friction.Generally, the hardness value of the selected material is 70 Shore hardness, and the selection of compressibility has been discussed in previous article. A and L are only related to the size of the structure.
The reason why friction forth generated by the O-ring seal is so important is that it has an impact on the specific pressure of the end faces of the mechanical seal.Improper design will cause the mechanical seal to fail. When calculating the end surface specific pressure, the existence of friction specific pressure should be considered. The formula is as follows:
Pc = Ps + PL( K － λ)-PF
Pc is the specific pressure of the end face, MPa; Ps is the specific pressure of the spring, MPa; PL is the medium pressure, MPa;K is the load coefficient; λ is the back pressure coefficient. PF is the friction specific pressure (unit: MPa), and the value can be obtained by dividing Ff(friction force of O-ring seal) by Af (contact area of mechanical seal).
Because load coefficient is close to back pressure coefficient, end surface specific pressure (positive and negative) mainly depends on spring specific pressure and friction specific pressure.
When friction specific pressure is greater than spring specific pressure, end surface specific pressure is negative, and the mechanical seal surface Will be opened and will fail. Therefore, the general friction specific pressure must not exceed 0.02 MPa.
In summary, friction forth of O-ring seals cannot be ignored in design of mechanical seals. It must be calculated and checked, otherwise the entire mechanical seal will fail.
Friction forth of O-ring seal includes adhesive friction and hysteresis friction forth(s). Hysteresis friction force is smaller than adhesive friction force, so when estimating the influence of friction force of O-ring seal on the rotary face and stationary seat, it is safer to use adhesive friction force for calculation.
Adhesive friction forth is composed of two parts, and the calculation formula is as follows:
Fa = Fc L + FH Ap
Fa is adhesive friction force, measuring unit is N；Fc is compression friction coefficient of the O-ring seal, N/m；FH is pressure friction coefficient of the O-ring seal, MPa； L is the contact length of the O-ring seal, m ;Ap is the projected area of the O-ring seal after assembly, m2
Among them,Fc and FH have little relevance to type of material, but are related to hardness and compression rate of the material, and can only be measured through experiments.The greater the hardness of the material, the lower the friction; the lower the compression rate, the lower the friction.Generally, the hardness value of the selected material is 70 Shore hardness, and the selection of compressibility has been discussed in previous article. A and L are only related to the size of the structure.
The reason why friction forth generated by the O-ring seal is so important is that it has an impact on the specific pressure of the end faces of the mechanical seal.Improper design will cause the mechanical seal to fail. When calculating the end surface specific pressure, the existence of friction specific pressure should be considered. The formula is as follows:
Pc = Ps + PL( K － λ)-PF
Pc is the specific pressure of the end face, MPa; Ps is the specific pressure of the spring, MPa; PL is the medium pressure, MPa;K is the load coefficient; λ is the back pressure coefficient. PF is the friction specific pressure (unit: MPa), and the value can be obtained by dividing Ff(friction force of O-ring seal) by Af (contact area of mechanical seal).
Because load coefficient is close to back pressure coefficient, end surface specific pressure (positive and negative) mainly depends on spring specific pressure and friction specific pressure.
When friction specific pressure is greater than spring specific pressure, end surface specific pressure is negative, and the mechanical seal surface Will be opened and will fail. Therefore, the general friction specific pressure must not exceed 0.02 MPa.
In summary, friction forth of O-ring seals cannot be ignored in design of mechanical seals. It must be calculated and checked, otherwise the entire mechanical seal will fail.
