Parametric Testing

Strategies and Devices for Parametric Testing of Radial Lip Seals

Radial Lip Seal Function, Construction, and Materials

Radial Lip Seal Physics

Why Parametric Testing?

Parametric Testing Devices for Radial Lip Seals

Results: Seal Characterization

Conclusions

Radial Lip Seal Function and Kinematics

Function: Separates a contained liquid from the external environment while transmitting continuous rotational movement about its axis of symmetry. Tolerates, without leakage, low-amplitude perturbations in the remaining five degrees of freedom.
Shown: Deflected cross-section ...

The Physics of Radial Lip Seals

Parameter

Governing Equation
Measurable Quantity Scaling Law
(Rule of Thumb) Typical Range

Hydrodynamic Pumping Reynolds: flow, cc/min (q) 0.05 - 1.00

Radial Load Hoop Stress:
line force, lb/ circumf in (l) 5 - 25

Eccentric Load force, lb (W_e) <2Dl

Static Leakage Molecular Scale Laminar Viscous Flow flow, cc/hr Viscosity on scale of molecular length 10 X normal 0 - 0.01

Why Do Parametric Testing?

Parametric testing complements time-based endurance testing

Provides immediately useful design data

Validates analysis

Is essential for life prediction

Is fast (minutes instead of months)

Is inexpensive (Only one test rig is required, rather than three and up for each test.)

Is unambiguous (If a part fails in endurance testing, the reason may not be clear.)

Downside: Data reduction may be complex (although it can be automated).

Parametric Testing Devices and Strategies

Test Device	General Description	Measurement Domain	Output
Hydrodynamic Pumping Rig	Device measures volume, as level change in a sight glass, pumped against a specified head in a specified time interval.	Time, fluid volume, pressure, fluid temperature, rotational speed, counterface diameter	Pumped volume as a function of time, fluid viscosity (temperature), speed, and head, normalized by counterface diameter.
Radial Load Fixture	Split mandrel equipped with load cell measures force to compress halves.	Force, time, counterface diameter	Radial load (lbs per inch of curcumference) as a function of time, max load, min load after specified time, decay rate.
Eccentric Load Rig	Bearing-mounted counterface capable of specified radial displacement and load measurement.	Force, time, radial displacement, angular location	Max eccentric load curve as function of angular location.
Static Leakage Fixture	Pressurized cavities, each equipped with a sight glass, measure contained fluid volume change over time, without rotation.	Time, fluid volume, pressure, temperature	Volume change over a specified interval as a function of time. Volume change is corrected for pressure, temperature, and trapped air volume.

Hydrodynamic Pumping Rig

Radial Load Fixture

Eccentric Load Rig

Static Leakage Fixture

Results — Seal Characterization

Qualitative seal pumping performance, q vs. time and q vs. N.

Results — Seal Characterization (continued)

Radial Load

Results — Seal Characterization (continued)

Eccentric loads -- wave form

Results — Seal Characterization (continued)

Eccentric loads -- linearization

Results — Seal Characterization (continued)

Static leakage -- Transparent mandrel shows unbroken contact patch.

Conclusions

Three important, but often elusive, radial lip seal design failure modes can be detected and eliminated by the use of parametric testing.

Static Leakage
Hydrodynamic pumping deficiency
Inadequate radial load

Design calculations can be verified quickly and with high confidence.

Optimum design solutions can be validated without long-term testing.

Parametric tests enable functional auditing as a quality assurance tool.

Parameter	Governing Equation	Measurable Quantity	Scaling Law (Rule of Thumb)	Typical Range
Hydrodynamic Pumping	Reynolds:	flow, cc/min (q)		0.05 - 1.00
Radial Load	Hoop Stress:	line force, lb/ circumf in (l)		5 - 25
Eccentric Load		force, lb (W_e)		<2Dl
Static Leakage	Molecular Scale Laminar Viscous Flow	flow, cc/hr	Viscosity on scale of molecular length 10 X normal	0 - 0.01