Garter Spring

Tags Wiki: #forces , #garter , #mechanics , #sealing , #seals , #spring .

Garter Spring describes garter spring as a practical tribology topic: how surfaces interact, how friction, wear, lubrication, material response, and operating environment influence performance, and what engineers should check when applying the concept in real machines or laboratory tests.

Core idea: garter spring should be interpreted as a system-level contact problem, not as an isolated material property.
Engineering relevance: load, speed, temperature, surface roughness, chemistry, and lubrication regime can change the observed behavior.
Good practice: combine measurements with surface inspection and clear reporting of test conditions.
Failure prevention: use the topic to identify risks early, compare alternatives, and improve reliability.

What is Garter Spring?

Garter Spring is part of the wider study of friction, wear, contact mechanics, materials, and lubrication. In practical use, the term is most useful when it is connected to a defined contact pair, surface condition, operating load, motion type, and environment.

For engineers, garter spring is not only a definition. It is a way to ask which mechanisms control performance at an interface: elastic or plastic deformation, adhesion, abrasion, fatigue, chemical film formation, lubricant starvation, debris generation, or changes in surface topography.

Why Garter Spring matters in tribology

Garter Spring matters because small changes at the surface can produce large changes in efficiency, temperature, noise, lifetime, and failure risk. A contact that appears acceptable under one load, speed, or lubricant can behave very differently when the regime shifts from full-film lubrication to mixed or boundary lubrication.

Question	Why it matters
What materials and surfaces are in contact?	Hardness, roughness, coating, oxide layers, and chemistry influence friction and wear.
What motion occurs?	Sliding, rolling, reciprocating, impact, and vibration activate different damage mechanisms.
What lubrication regime is present?	Boundary, mixed, hydrodynamic, and elastohydrodynamic regimes produce different surface separation and film behavior.
What environment is involved?	Temperature, humidity, vacuum, contamination, and reactive species can strongly change tribochemical response.

Engineering factors to consider

Contact stress and geometry: estimate nominal and local contact pressure, including asperity-scale effects where relevant.
Surface topography: measure roughness, waviness, texture direction, and surface defects before and after testing.
Lubricant selection: consider viscosity, additives, supply method, contamination, and compatibility with materials.
Thermal conditions: frictional heating can change viscosity, hardness, oxidation rate, and tribofilm formation.
Running-in: early surface adaptation may control the later steady-state response.
Debris control: wear particles can become a third body, accelerating abrasion or changing friction.

Measurement and interpretation

When evaluating garter spring, report the test configuration clearly: specimen geometry, material grade, surface roughness, lubricant, load, speed, temperature, duration, environment, and repeat count. Tribology results are often system-dependent, so a coefficient of friction, wear scar, or lifetime result should not be treated as a universal property without context.

Useful measurements include friction trace, wear volume or wear rate, surface profilometry, optical or electron microscopy, hardness, lubricant condition, and chemical surface analysis where tribofilms or oxidation are expected.

Detailed background and source material

A Garter spring is a coiled steel spring that is connected at each end to create a circular shape, and is used in oil seals, shaft seals, belt-driven motors, and electrical connectors. There are several ways to connect the ends depending on the application: connector ends, loop connection and tapered end connection. Garter springs are generally divided in compression and extensions springs according to the direction of the radial load exerted, i.e., inwards, or outwards.

Figure 1. Garter spring sketch.

Many lip seals count with a Garter spring for loading the seal tip against the shaft. The extra load provided by the Garter spring ensures that the stress relaxation of the seal material does not decay beyond the percolation limit of the contact. Because the load exerted by Garter spring is generally constant, i.e., the spring does not generally change its elongation while operating, Garter springs for shaft seals are often referred to as “constant load springs”.

Theoretical background

There is little written on garter springs. The following procedure estimates the radial load exerted by an extension garter spring when working under linear deflection:

1	Wire diameter	$d$
2	Mean diameter of the helix	$D$
3	Shaft diameter	$D_s$
4	Initial tension	$P_I$
5	Spring Working Length	$L_0$
6	Young’s modulus	$E$
7	Rigidity modulus	$G$

The following parameters can be deduced from the ones above:

8	Number of working coils	$n$
9	Inside diameter of the Garter spring	$D_r_i$
10	Increase in diameter when fitted $\Delta D = D_s - D_r_i$	$\Delta D$
11	Spring rate	$S$
12	Spring Index $c = D/d$	$c$
13	Correction Factor $k =(c + 0.2)/(c-1)$	$k$
14	Combined stress due to bending and torsion	$q$
15	Additional stress due to initial tension	$q_I$
16	Total stress $q_t = q_I + q$	$q_t$
17	Circumferential force	$P_c$
18	Radial force unit per length	$P_r$

The number of working coils can be deduced from the working spring length $L$ :

$n = \frac{L_0}{d}$

Therefore, the inside diameter of the garter spring $D_r_i$ (when both edges are attached together):

$D_r_i =\frac{L_0}{\pi } = \frac{nd}{\pi } - d^*$

(*sometimes $d/2$ is used instead. The difference between both option is usually not significant.)

The spring rate $S$ is obtained from the following formula:

$S =\frac{d^4G}{8nD^3} = \frac{dG}{8nc^3}$

The circumferential and radial loads are related by the following expression (see section below):

$P_r =\frac{2P_c}{D_s}$

The force exerted radially by the garter spring when mounted is defined as follows:

$P{_c__{extension}} = P_I + S(\pi D_s-\pi D_r_i)$

$P{_r__{extension}} = 2(\frac{P_I}{D_s}+\pi S(1-\frac{D_r_i}{D_s}))$

$P{_c__{compression}} = S(\pi D_s-\pi D_r_i)$

$P{_r__{compression}} = 2(\pi S(\frac{D_r_i}{D_s}-1))$

Most extension springs are wound with initial tension $P_I$ . This is an internal force that holds the coils tightly together. Unlike a compression springs, which has zero load at zero deflection, an extension spring have an initial tension $P_I$ .

Figure 2. Load-deflection curve for helical spring with initial tension $P_I$ .

It is important to estimate the total stress imposed on the spring due to torsion and bending $q_t$ . By comparing it with the yield strength of the spring material $\sigma_y$ it is possible to check if the spring works in the elastic deformation range.

The stress resultant from the spring elongation $q$ is estimated as follows:

$q=(\frac{D_s-D_r_i}{D}+\frac{2}{1+\frac{2G}{E}}) \frac{Gk}{nc}$

It is necessary to include the stress induced by the initial tension $q_I$ :

$q_I = \frac{8cP_Ik}{\pi d^2}$ :

The relaxation of the spring material is usually neglected but it sould be considered when operating at elevated temperatures.

Relationship between the circumferential and radial loads

Figure 3. Radial and circumferential load decomposition.

To calculate the radial load exerted by a Garter spring, the Hooke’s law is applied in the circumferential direction and the radial load is obtained from forces decomposition:

$P_r [\frac{N}{m}] = \frac{2P_c}{D_2} = \frac{2}{D_2}(P_I+k\pi (D_2-D_1))$

Realtionship between radial $P_r [\frac{N}{m}]$ and circumferential $P_c [N]$ forces

$P_c [N] = $\int_{s_1}^{s_2} P_r sin(\theta) \, ds$$

$P_c [N] = $\int_{0}^{\pi/2} P_r sin(\theta) \frac{D}{2}\, d\theta$$

$P_c [N] = P_r \frac{D}{2} [-cos (\theta)]_0^{\pi/2} = P_r \frac{D}{2}$

$P{_{r__{tot}}} [N]= P_r [\frac{N}{m}] \pi D [m]$

FAQs

What is Garter Spring in simple terms?

Garter Spring is a tribology topic used to understand how contacting surfaces behave under load, motion, lubrication, and environmental conditions.

Why is garter spring important for engineers?

It helps engineers diagnose friction and wear problems, choose materials or lubricants, design tests, and prevent surface-related failures.

How should garter spring be tested?

Testing should reproduce the relevant contact geometry, motion, load, speed, temperature, surface roughness, lubricant, and environment as closely as practical.

What should be reported with garter spring results?

Report materials, roughness, lubricant, load, speed, temperature, duration, environment, friction history, wear measurement method, and post-test surface observations.

References and further reading

Bhushan, B. Introduction to Tribology. Wiley.
Stachowiak, G. W., and Batchelor, A. W. Engineering Tribology. Butterworth-Heinemann.
Hamrock, B. J., Schmid, S. R., and Jacobson, B. O. Fundamentals of Fluid Film Lubrication. CRC Press.
NIST resources for measurement, materials, and surface characterization.

Last updated: May 2026. Reviewed topic: Garter Spring, tribological performance, measurement, and failure prevention.