Failure Modes of Mechanical Springs

Failure Modes of Mechanical Springs William H. Skewis Support Systems Technology Corporation Mechanical springs are used in machine designs to exert force, provide flexibility, and to store or absorb energy. Springs are manufactured for many different applications such as compression, extension, torsion, power, and constant force. Depending on the application, a spring may be in a static, cyclic or dynamic operating mode. A spring is usually considered to be static if a change in deflection or load occurs only a few times, such as less than 10,000 cycles during the expected life of the spring. A static spring may remain loaded for very long periods of time. The failure modes of interest for static springs include spring relaxation, set and creep. Cyclic springs are flexed repeatedly and can be expected to exhibit a higher failure rate due to fatigue. Cyclic springs may be operated in a unidirectional mode or a reversed stress mode. In one case, the stress is always applied in the same direction, while in the other, stress is applied first in one direction then in the opposite direction. Figure 1 shows the difference in deflection and stress between these two operating modes. For the same maximum stress and deflection between a unidirectional and reversed stress spring, the stress range for the reversed stress spring will be twice that of the unidirectional spring and therefore a shorter fatigue life would be expected. Dynamic loading refers to those intermittent occurrences of a load surge such as a shock absorber inducing higher than normal stresses on the spring. Dynamic loading of a spring falls into three main categories: shock, resonance of the spring itself, and resonance of the spring/mass system. Shock loading occurs when a load is applied with sufficient speed such that the first coils of the spring take up more of the load than would be calculated for a static or cyclic situation. This loading is due to the inertia of the spring coils. Spring resonance occurs when the operating speed is the same as the natural frequency of the spring or a harmonic of the natural frequency. Resonance can cause greatly elevated stresses and possible coil clash resulting in premature failure. Resonance of the spring/mass system occurs when the spring is required to carry a mass attached to its moving end and the combined system is subject to resonance at a much lower cycle rate than the spring alone. Failure modes for dynamic loading of a spring include fracture of the spring material due to shock pulses and resonance surging. Springs tend to be highly stressed because they are designed to fit into small spaces with the least possible weight and lowest material cost. At the same time they are required to deliver the required force over a long period of time. The reliability of a spring is therefore related to its material strength, design characteristics, and the operating environment. Most springs are made of steel and material strength of the spring is usually listed in terms of tensile strength in relation to the expected spring stress. Corrosion protection of the spring steel has a significant impact on reliability and therefore material properties, the processes used in the manufacturing of the spring, operating temperature, and corrosive media must all be known before any estimate of spring reliability can be made. Spring reliability is also directly related to

the surface quality and the distribution, type and size of sub-surface impurities in the spring material. Common materials of construction for springs include spring steel, stainless steel, nickel base alloy, and copper base alloy or bronze. Spring steel is any variety of steels that are normally of the high-carbon or alloy type. High carbon spring steels are probably the most commonly used material for springs except for those to be used in high or low temperature environments or for shock or high impact loads.

Unidirectional Stress

Reversed Stress Figure 1. Cyclic Modes of Spring Operation 2

FAILURE MODES Springs of all types are expected to operate over very long periods of time without significant changes in dimension, displacement or spring rate, often under changing loads. Considering these requirements, potential failure modes include yielding, fatigue, corrosion fatigue, fretting fatigue, creep, thermal relaxation, buckling, and force-induced elastic deformation. The operating life of a mechanical spring arrangement is dependent upon the susceptibility of the materials to corrosion and stress levels (static, cyclic or dynamic). The most common failure modes for springs are fracture due to fatigue and excessive loss of load due to stress relaxation. By definition, objects that are loaded under purely oscillatory loads (σmean = 0) fail when their stresses reach the material’s fatigue limit σfatigue. Conversely, objects that are loaded under purely static loads (σalt = 0) fail when their stresses reach the materials yield limit σyield. For springs that have a mixture of σmean and σalt stresses, the Soderberg Criterion provides a way to calculate a failure limit. Mean stress is plotted on one axis and alternating stress on the other. Figure 2 is a typical Soderberg plot.

Figure 2. Soderberg plot

Fatigue Stress: All springs have finite fatigue limits, the limit depending fatigue stress and the degree of fluctuating loads. The four most common fatigue stress conditions include constant deflection, constant load, unidirectional stress and reversed stress. A spring inside a valve assembly is an example of a constant deflection where the spring is cycled through a specified deflection range. An example of a constant load spring is the use of vibration springs under a dead weight where the load applied to the spring does not change during operation but the deflection will. A unidirectional stress is one where the stress is always applied in the same direction such as used in the return spring of an actuator. A reversed stress is applied first in one direction then in the opposite direction such as used in a regulator valve. The three stages to a fatigue failure include crack initiation, crack propagation and finally fracture of the spring material.

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Static springs can be used in constant deflection or constant load applications. A constant deflection spring is cycled through a specified deflection range, the loads on the spring causing some set or relaxation which in turn lowers the applied stress. The spring may relax with time and reduce the applied load. Under constant load conditions, the load applied to the spring does not change during operation. Constant load springs may set or creep, but the applied stress is constant. The constant stress may result in fatigue lives shorter than those found in constant deflection applications. A corrosive environment may accelerate the time to fatigue failure, corrosion reducing the load-carrying capability of a spring and its life. The precise quantitative effect of a corrosive environment on spring performance is difficult to predict. Springs are almost always in contact with other metal parts. If a spring is to be subjected to a corrosive environment, the use of inert materials provides the best defense against corrosion. Protective coatings can also be applied. In special situations, shot peening can be used to prevent stress corrosion and cathodic protection systems can be used to prevent general corrosion. The spring material is normally more noble (chemically resistant to corrosion) than the structural components in contact with it because the lesser noble alloy will be attacked by the electrolyte. The effects of corrosion on spring reliability must be based on experience data considering the extent of a corrosive environment. Surging (resonant frequency response) can occur in high-speed cyclic applications if axial operating frequencies approach the axial natural frequency of the helical-coil spring. If the material and geometry of the axially reciprocating spring are such that its axial natural frequency is close to the operating frequency, a traveling displacement wave front is propagated and reflected along the spring with about the same frequency as the exciting force. This condition results in local compressions and rarefactions producing high stresses and/or erratic forces locally, with consequent loss of control of the spring-loaded object. Surging of a valve spring, for example may allow the valve to open erratically when it should be closed or vice versa.

Spring Relaxation: Springs of all types are expected to operate over long periods of time without significant changes in dimension, displacement, or spring rates, often under fluctuating loads. If a spring is deflected under full load and the stresses induced exceed the yield strength of the material, the resulting permanent deformation may prevent the spring from providing the required force or to deliver stored energy for subsequent operations. Most springs are subject to some amount of relaxation during their life span even under benign conditions. The amount of spring relaxation is a function of the spring material and the amount of time the spring is exposed to the higher stresses and/or temperatures. Static springs can be used in constant deflection or constant load applications. A constant deflection spring is cycled through a specified deflection range, the loads on the spring causing some set or relaxation which in turn lowers the applied stress. The spring may relax with time and reduce the applied load. Elevated temperatures can cause thermal relaxation, excess changes in spring dimension or reduced load supporting capability. Under constant load conditions, the load applied to the spring does not change during operation. Constant load springs may set or

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creep, but the applied stress is constant. The constant stress may result in fatigue lives shorter than those found in constant deflection applications. In many applications, compression and extension springs are subjected to elevated temperatures at high stresses which can result in relaxation or loss of load. This condition is often referred to as "set". After the operating conditions are determined, set can be predicted and allowances made in the spring design. When no set is allowed in the application, the spring manufacturer may be able to preset the spring at temperatures and stresses higher than those to be encountered in the operating environment. A highly stressed spring will set the first several times it is pressed. Relaxation is a function of a fairly high stress (but usually lower then that required to cause set) over a period of time. Creep in the spring may lead to unacceptable dimensional changes even under static loading (set). A spring held at a certain stress will actually relax more in a given time than a spring cycled between that stress and a lower stress because it spends more time at the higher stress. The amount of spring relaxation over a certain period of time is estimated by first determining the operating temperature, the maximum amount of stress the spring sees and how long the spring will be exposed to the maximum stress and the elevated temperature over its lifetime.

Miscellaneous Failure Modes: Most extension spring failures occur in the area of the spring end. Extension springs are designed to become longer under load and their maximum length must be controlled for long life. Their turns are normally touching in the unloaded position and they have a hook, eye or some other means of attachment. For maximum reliability, the spring wire must be smooth with a gradual flow into the end without tool marks, sharp corners or other stress risers. The spring ends should be made as an integral part of the coil winding operation and the bend radius should be at least one and one-half times the wire diameter. Other failure mechanisms and causes may be identified for a specific application to assure that all considerations of reliability are included in the prediction. For example, another failure mode to be considered is the hardness of the spring material that can be sensitive to plating and baking operations. Table 1 is a list of failure modes and causes for mechanical springs.

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Table 1. Typical Failure Modes and Causes for Mechanical Springs TYPE OF SPRING/STRESS CONDITION

FAILURE MODES

FAILURE CAUSES

- Static (constant deflection or constant load)

- Load loss - Creep - Set - Yielding

- Parameter change - Hydrogen embrittlement

- Cyclic (10,000 cycles or more during the life of the spring)

- Fracture

- Excessive mean stress operation * - Material flaws - High temperature operation - Imperfection on inside diameter of the spring - Hydrogen embrittlement - Stress concentration due to tooling marks and rough finishes - sharp bends on spring ends (extension springs) - Surface imperfections (high cycle with no shot peening) - Corrosive atmosphere - Misalignment - Excessive stress range of reverse stress ** - Cycling temperature - Low frequency vibration - High frequency vibration - Maximum load ratio exceeded - Insufficient space for operation - Shock impulse - Surface defects - Excessive stress range of reverse stress

- Damaged spring end - Fatigue failure - Buckling - Surging - Complex stress change as a function of time

- Dynamic (intermittent occurrences of a load surge)

- Fracture - Fatigue failure

- Resonance surging * Unidirectional Operation ** Reverse Stress Operation

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FAILURE MODE AND EFFECTS ANALYSIS (FMEA) WORKSHEET This section provides some guidelines for performing the FMEA for mechanical springs. The FMECA is normally performed by first identifying the failure modes and then estimating the probability of occurrence for each identified failure mode. Failure modes have been discussed in the previous section. Their probability of occurrence depends on the material strength with respect to operating stress which in turn depends to a large extent on the following items: - Type of spring (compression, extension, torsion, etc.) - Size of spring (diameter, length) - Spring material (Monel, music, high-carbon, etc.) - Wire diameter - Loading (static, cyclic, dynamic) - Operating temperature - Spring design (spring index, bends, etc.) - Manufacturing methods & quality control (tool marks, pits) - Hook design - Operating environment (stress range) The following guidelines assume that a hardware bottom-up analysis is being performed. The purpose of the FMEA is to identify all potential failure modes of the equipment. Following the FMEA a Criticality Analysis (CA) is performed to identify the probabilities of occurrence. The CA will be discussed in the next section. The worksheet items of the FMEA are as follows: Function: Since the mechanical spring is generally a part of a component at a higher indenture level, the spring must be evaluated for reliability considering the operating environment of the valve, actuator, brake assembly or other component where the spring is located. Therefore, the spring should be identified as a functional description as shown in Figure 1 below. This process permits a technical review of the spring function within the equipment and identification of all failure modes applicable to the spring design.

Function No.

Function

Mode ID No.

1

1

Functional description of the spring at the next higher indenture level 2

Failure Modes

Cause ID No.

Failure modes of the spring such as wire fracture

Only one failure mode should be entered for each row because of individual failure effects, α and β values and compensating provisions

1

2 3 1

Failure Causes Description of all probable independent causes of this specific failure mode at the spring level such as corrosion Sharp wire bend Misalignment

2

Figure 1. Partial FMEA Worksheet 7

Failure Modes: It is important that the identified failure mode be directed toward a specific failure mode at the component level such as fracture. Only one failure mode should be entered for each row because of individual failure effects, α and β values and compensating provisions. Some failure modes will have a very low probability of occurrence or may not be applicable for a given application. They can be either eliminated from the worksheet or a 0.0 probability of occurrence assigned. Failure Causes: The different causes of the failure mode occurring should be entered one row at a time so that the probability of occurrence of each failure cause can be entered. The operating environment needs to be considered so that all possible causes are entered such as hydrogen embrittlement, corrosive fluid leakage, pressure pulsations and imperfections on the inside of the spring coil. Failure Effects: Local failure effects are the consequences of the failure mode on the equipment at the part level. This entry may be the same as the failure cause or the failure effect at the component level depending on the complexity of the total equipment. The effect of the failure mode occurrence at the next higher indenture level is then entered. See Figure 2. The end failure effect is the consequence of the failure mode occurrence at the total system level. In some cases the failure effects for different failure causes will be the same. For example, random dynamic loading of a spring can cause unexpected opening or closure of a valve assembly or movement of an actuator caused by shock.

Mission Phase or Operational Mode

The mode in which the equipment is operating when the spring fails

Local Failure Effect

Next Higher Failure Effect

Consequences of the failure cause on equipment operation at the part level

Impact of the failure at the next higher indenture level

End Failure Effect

Total effect of the failure mode on the operation, function or operational status

Failure Detection Method Method by which occurrence of the failure cause is detected

Figure 2. Typical FMEA Worksheet (continued)

Failure Detection Method: The method by which occurrence of the failure cause is detected is entered on the worksheet which may be by many different means such as operator, warning device, sensing device or routine diagnostic maintenance. Compensating Provisions: Provisions in the design or actions by an operator are entered on the worksheet that circumvent or mitigate the effect of the failure such as equipment redundancy, alarm provisions, safety features or alternate modes of operation. For example, if a compression

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spring fails, catastrophic failure of the supported assembly is often prevented by the fact that the parts containing the ends of the compression spring will simply compress the spring. An extension spring on the other hand may have no travel limit since the spring is in tension and the maximum stress can result in spring damage. Figure 3 shows a continuation of a typical FMEA worksheet. Severity Class: One of the following severity categories is recorded: Class 1 is catastrophic, a failure which may cause death, total property damage or loss of mission (fire). Class II is critical, a failure that may cause severe injury or extensive property damage (transmission failure). Class III is marginal, a failure that may cause minor injury, minor property damage or mission degradation (excessive leakage). Class IV is minor (a failure not serious enough to cause injury, property damage or mission degradation but may necessitate repairs at a later time (minor leakage). Remarks: Remarks need to be entered that clarify or amplify other worksheet entries such as design features, safety provisions combination of entries, etc.

Compensating Provisions

Severity Class

Remarks

Provisions in the equipment design which alter the effect of failure mode occurrence

Severity Class of I, II, III, or IV

Remarks that clarify or amplify worksheet entries

Figure 3. Typical FMEA Worksheet (continued)

CRITICALITY ANALYSIS (CA) WORKSHEET A Criticality Analysis (CA) is performed following the FMEA to determine the probability of occurrence that an identified failure mode will actually result in the defined end effect. In most cases the CA worksheet will repeat the columns that identify the component function, failure modes, failure causes, and ID numbers so that the entries can be traced from one worksheet to another. See Figure 4. This section provides some guidelines for determining failure rates and occurrence probabilities.

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Failure Rate Data Source

OREDA, RAC, NPRD, NSWC HDBK, etc.

Base Failure Rate

Failure rate from source

Adjustment Factors (π)

Correction factors as required to convert acquired failure rate to application failure rate

Part Failure Rate

Failure Effect Probability (β)

Failure Mode Ratio (α)

Base failure rate multiplied by π factor

Conditional probability that identified failure mode will result in end effect (0.0 to 1.0)

Ratio of part failure rate related to identified failure mode

Figure 4. Partial CA Worksheet Failure Rate Data Source: The source of the part failure rate is entered such as RAC, OREDA, NPRD or NSWC Handbook. In some cases an S10 value for the spring can be obtained from the manufacturer. The S10 value is the number of cycles that 90% of the springs operating at the published stress level can be expected to complete or exceed before exhibiting the first evidence of fatigue. Base Failure Rate: The failure rate of the part as provided by the data source. Before entering the failure rate on the worksheet, it needs to be reviewed in terms of the material strength, application and projected use environment with respect to the spring being analyzed. Typical failure rate considerations include: level of loading, operating temperature, cycling rate and corrosive environment. The base failure rate can be altered with the adjustment factor based on individual experience with similar designs. Adjustment (π) factor: If the failure rate is obtained from a source where the operational environment is different from the intended operating environment of the equipment being analyzed, an adjustment factor may need to be applied. A return spring for example may be a cyclic spring as part of a sliding-action dynamic seal. Depending on the failure rate source, the duty cycle may need to be adjusted for the specific application. For example, an extension spring can be expected to have a higher failure rate than an equivalent compression spring, a reverse stress spring will have a higher failure rate than a unidirectional spring, and a constant load spring can be expected to have a higher failure rate than a constant deflection spring. Part Failure Rate: This entry is simply the base failure rate multiplied by the π factor. Failure Effect Probability: The failure mode may directly result in the listed end effect or it may not always result in the listed end effect. It is an engineering judgment as to the probability of occurrence after reviewing the failure causes and the operating environment of the equipment. Failure Mode Ratio: The part failure rate will be the total failure rate of the component containing the spring under normal operating conditions. This failure rate needs to be subdivided per the particular component failure rate. The total of all the α values for each equipment item (function) entry will be equal to 1.0.

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Failure Mode Criticality Number: This number is a combination of the part failure rate, failure effect probability and failure mode ratio. This number establishes a reference that can be used to compare the occurrence probability of this particular failure mode with other failure mode probabilities. See Figure 5. Part Criticality Number: A criticality number for the part is sometimes valuable for evaluating the criticality of the particular valve being analyzed in relation to other components of the system. The part criticality number is simply the summation of all the failure mode criticality numbers for the component. Severity Class: The severity class from the FMEA worksheet is normally repeated for informational purposes. Remarks: Remarks are included on the worksheet to clarify or amplify other entries on the worksheet such as explanations of failure rate sources, adjustment factors, modifications and calculations.

Failure Mode Criticality Number, CN (λp x β x α)

Part Criticality Number ∑ CN

Severity Class

Remarks

Failure mode criticality number establishes reference for comparison purposes

Part criticality number establishes reference for comparison purposes

The severity of the failure mode copied from the FMEA worksheet

Remarks to clarify or amplify other worksheet entries such as failure rate sources and adjustment factors

Figure 5. Typical CA Worksheet (continued)

SUMMARY The “Handbook of Reliability Prediction Procedures for Mechanical Equipment” provides the procedures for performing a FMECA and determining the component failure rates. The Handbook along with a supporting software package can be downloaded free of charge. The Handbook was developed by the Carderock Division of the Naval Surface Warfare Center. Support Systems Technology Corp. (SSTC) was the primary contractor on this development project and we continue to provide research in maintaining the Handbook and the supporting MechRel software program. To obtain a complete copy of the Handbook and the MechRel software package at no cost, contact us at [email protected] SSTC is an engineering consulting firm. We can also provide reliability engineering support for your development project. If you have any questions concerning the Handbook or our

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capabilities in performing a complete analysis for your equipment, contact us at [email protected] . To return to the MechRel home page, click here: http://www.mechrel.com/

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