Tolerance Requirements of Mechanical Seals: How To Control Leakage

Mechanical seals, as one of the most critical sealing components in rotating equipment, are widely used in structures such as pumps, compressors, gearboxes, and reactors. Their function is to prevent the leakage of media (such as oil, water, and gas) along the rotational axis. Due to the complex operating conditions where the sealing face alternates between high-speed, micro-gap, contact, and non-contact states, the tolerance design of the sealing parts directly affects the reliability and service life of the equipment.

This paper will start with the analysis of mechanical seal structure, systematically explain the key points of tolerance control of mechanical seal components, and give typical matching scheme and assembly suggestions.

Brief description of the structure and working principle of mechanical seal

Mechanical seal is mainly composed of two core components:

Motion ring (Rotary Ring): rotates with the shaft

Static ring (Stationary Ring): Fixed in the housing and not moving

The two form a dynamic sealing pair through the flat sealing end face, and maintain a micro-contact state with the fluid film by relying on the pre-tension spring force to achieve sealing.

The gap between the sealing surfaces is usually in the range of 0.3~1μm. Any tolerance exceeding the limit may lead to:

Seal failure → medium leakage

Sealing too tight → wear too fast, heat, sintering

Key points of tolerance control for mechanical sealing components

1. The fit between the shaft and the moving ring seat

Recommended pairing: h6/k5 or h6/m5

The motion ring is required to rotate synchronously with the shaft without slipping

Excessive overfitting → difficult installation, stress concentration

If the fit is too small, the moving ring will slip and the seal will fail

2. The outer circle of the static ring is matched with the hole of the shell

Recommended pairing: H7/n6 or H7/p6

It is required to be sealed and stably positioned, and cannot rotate or eccentric

If an elastic O-ring is used for cushioning, the fit must be tight to prevent movement

3. Sealed end face jump control

Maximum allowable axial jump: ≤ 3~5μm

Circular runout affects the fit of the sealing surface and causes the sealing film to be unstable

It is suggested to ensure the precision of machining and grinding through dynamic balance processing

Sealing problems and cases caused by poor tolerance

Typical structural tolerance fit reference

Assembly recommendations for sealed tolerance control

The sealing end face must be cleaned and lubricated before installation to avoid dry friction

The moving ring and static ring shall not be struck with a hammer, but shall be used with special guiding tools

The tolerance fit should be ensured to have no burrs, scratches and eccentric clamping

Dynamic balance correction is recommended, especially at high speeds (> 3000rpm)

The sealing performance of mechanical seal is highly dependent on axial runout control and interference design

Every slight deviation from the mating tolerance is multiplied into a leakage risk

Selecting the appropriate matching grade and paying attention to surface roughness and machining shape error are the core strategies to ensure reliable sealing

It is recommended to make a list of sealing tolerances in the structural design stage and set up running test points and acceptance standards in the assembly process