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10 February 2026

12 Pages

An Adverse Outcome Resulting from an Aftermarket Modification of a Suspension Seat: A Sentinel Health Event Investigation

1
Center for Family and Community Medicine, Columbia University, New York, NY 10032, USA
2
Johanning MD PC, Albany, NY 12203, USA
Vibration2026, 9(1), 11;https://doi.org/10.3390/vibration9010011
This article belongs to the Special Issue Whole-Body Vibration and Hand-Arm Vibration Related to ISO-TC108-SC4 Published Standards
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Abstract

In a sentinel health event investigation of a back disorder claim, the vibration exposure and ergonomic function of a modified suspension seat were assessed. Background: In a forensic occupational injury investigation, an aftermarket-altered operator seat in a railroad rail-track tamper machine was evaluated. Methods: Detailed whole-body vibration (WBV) exposure measurements were conducted according to current applicable technical standards and guidelines (i.e., ISO 2631-1:1997) on a 09-16 DYNACAT Continuous Action Tamper with Stabilizer during routine track repair services. The modified Grammer Mfg. suspension operator seat was evaluated for performance and ergonomic features (i.e., adjustability, posture, and suspension quality). Results: The tested seat appeared to underperform and was overloaded with the aftermarket control devices, attachments and modifications. The suspension system’s end-stopper was damaged. The seat system had excessive play and wobbles; it was not firmly braced and attached. The vector sum (av) results ranged from 0.26 m/s2 (no tamping) to a maximal 0.55 m/s2 (tamping). The seat transfer (SEAT) analysis showed magnification of vibration input and variable performance of the suspension depending on operational tasks. Conclusions: The modified suspension seat underperformed and seemed to magnify and worsen the vibration, jolts and shock exposures of the seated operator. The heavy and bulky seat modifications likely limited the suspension function. The malfunctioning seat was more likely than not a contributing factor in the pathogenesis of the spinal disorders of the injured machine operator.

1. Introduction

A mitigation strategy to reduce potentially harmful exposure to vibration and shocks is the use of suspension seats in vehicles that expose the operator/driver to whole-body vibration (WBV) exposure. Suspension seats reduce whole-body vibration (WBV) by limiting the transfer of vibrations from the vehicle chassis to the operator using mechanical or active systems. Passive seats like air-ride models use air bladders or springs to dampen vertical (z-axis) vibrations, moderately reducing WBV, but are less effective against multi-axial vibrations common in off-road and mining settings [1,2,3]. Well-chosen and adjusted suspension seats help reduce WBV-related health risks like back pain and musculoskeletal disorders for drivers and operators [4]. A dedicated industry has engineered advanced, tailor-made passive and active suspension seats for applications in trucks, buses, off-road vehicles, rail transport, and cranes, as well as construction and maintenance-of-way vehicles. Vehicles may be equipped with suspension seats by vehicle manufacturers or retrofitted to replace old seats by the vehicle owner/user, but a mismatch and design errors may contribute to increased vibration exposure to the operator. Ideally, the seat should be matched to the vehicle characteristics and operator/driver tasks, and provide ergonomic features and adjustments as well as driver comfort while safely attenuating input vibration and shocks. The seat should be designed to the lowest technically achievable vibration transfer considering the dominant frequency of the vehicle. This sentinel health event investigation was related to a Federal Employers Liability Act (FELA) injury claim of a maintenance-of-way tamper operator who suffered from a lower back disorder. A sentinel event is an unexpected workplace incident causing or risking death, serious injury, or psychological harm. Such events require immediate investigation to identify, recognize and address potential process flaws or working conditions. An occupational sentinel health event refers to a disease, disability, or untimely death that is associated with workplace exposures. The occurrence of such an event may prompt epidemiological or industrial hygiene investigations and can provide valuable information for enhancing engineering controls and personal protective measures [5,6,7]. The worker reported that the seat in a tamper railroad machine was causing discomfort, pain and harm and claimed that the seat was defective. A tamping machine or ballast tamper is a self-powered rail vehicle designed to accurately adjust the position of railway tracks. It does this by using mechanized tines to lift ties and align the track. These machines use hydraulic and mechanical processes to pack and align ballast using vibration under railway tracks to provide stability and safe, reliable train operations. The tamping machine work cycles include the tamping of ties, leveling of track, correcting horizontal alignment, and packing of ballast in a single pass all controlled by the operator and the automated programmable logic controllers.

2. Materials and Methods

Detailed whole-body vibration (WBV) exposure measurements were conducted according to current applicable technical standards and guidelines (i.e., ISO 2631-1:1997 [8], ISO/EN 8041-1:2017 [9] ANSI and ACGIH, etc.) on a 09-16 DYNACAT Continuous Action Tamper with Stabilizer (built by Plasser American Corp (Chesapeake, VA, USA). in 2001, rebuilt 2015, Serial No. 3155; weight approximately 70 tons) operated by a Class I railroad in the USA during track repair service, using vibration measurement equipment to assess occupational exposure and potential health risks. WBV measurements were conducted during the night in Texas, USA, as part of a FELA claim investigation. The tested operator seat of the tamper was evaluated for ergonomic features (i.e., adjustability, posture, and suspension quality). The operator seat spring system was manufactured by Grammer, AG (Amberg, Germany). (labeled: MSG85 (Inventory 1237860/Order DE115978360019/assembly 2014/32)) and assembled with modifications by J.R. Merrit Controls, Inc. (Stratford, CT, USA). (labeled: Part No LRC100-14649 Job No 69894, SF32516 1/21/2015 Serial No. 11488877) to allow for one-operator control of the tamper. The Grammer seat had a vertical and longitudinal shock absorber as well as a weight adjustment spring suspension system. The rotating seat had weight, height, fore-after adjustments for the operator, a reclining backrest, a headrest and on the left- and right-side consoles with adjustable rests, switches, knobs, handles and four monitors on both sides (see Figure 1 and Figure 2). The consoles and monitors were attached between the suspension system and seat pad and back rest (see Figure 3). The operator has a seat height, fore/after adjustment. The vibration was measured utilizing the Svantek SV106 Human Vibration Meter & Analyzer (Svantek, Warsaw, Poland), Ref. [10] serial No 46292, the seat pad accelerometer Svantek SV 38A, serial No 43850, and, for the floor mounted accelerometer, Svantek SV151, serial No 31377; all devices are compliant with the measuring standards ISO 2631-1:1997 and ISO/EN 8041-1:2017. The vibration acceleration measured on top of the seat cushion reflects the vibration exposure transmitted to the seated machine operator (Figure 1 and Figure 4) and was compared to the seat mounting platform. The vibration acceleration measured at the seat mounting point represents the vibration excitation caused by the machine and interaction of the track and surface conditions. The ratio of the weighted vibration accelerations (r.m.s. = root mean squared values) measured on the seat and at the seat mounting point is expressed as the Seat Effective Amplitude Transmissibility (SEAT) according to ISO 10326-1:2016 [11]. Vehicle speed and location were monitored with a Garmin GPSMap 67i device (Garmin, Olathe, KS, USA). The railroad and operator consented to the measurement and audio-visual monitoring during a routine work cycle. The measuring devices and system were calibrated and checked to Svantek calibration factory conformity (available upon request). The collected data (02/2025) was analyzed with Svantek SvanPC++ software 3.5.2 (2025) in accordance with ISO 2631-1:1997, as shown in Appendix A [12].
Figure 1. Operator seat system with attached side consoles and monitors showing seat pad sensor location (view from top) (© E. Johanning).
Figure 2. Operator using hand and foot controls of chair system with side monitors attached to consoles (front/side view) (© E. Johanning).
Figure 3. Operator seat spring system (rear view, bellows partially removed) with metal attachment bar of the side consoles mounted between seat pad/back rest and spring system and fixated on rotating platform (location of floor accelerometer) (© E. Johanning).
Figure 4. Basicentric axes of the human body (ISO 2631-1:1997, Figure 1) and seat accelerometer location on seat surface.

3. Results

The railway tamping machine used pick-shaped tines that move in a vibrating motion and are powered by hydraulics that caused various vibration intensity. As it travels along the track, these tines repeatedly pressed into the ballast, compacting and tightening the ballast stones beneath the ties through dynamic vibration while exposing the operator to WBV. The vibration measurements on the rotating pedestal and seat pad were done during the entire work cycle of the machine operator, including moving to the work location, set-up of the machine operation, tamping and other routine work procedures. A total of 2 h of tamping data was recorded for this analysis. Data was evaluated using measurement segments of 15 min periods from the data logger files in SVAN PC++ software 3.5.2 (2025), and the results are reported below (Table 1). Due to the multi-axis vibration exposure to the machine operator, the vector sum is likely a better expression of the seat vibrations. The “total vibration value” or vector sum (av) [av = √[(1.4 awx)2 + (1.4 awy)2 + (awz)2] has been proposed for evaluation with respect to health and safety if no dominant axis of vibration exists (ISO 2631-1:1997 (E), 6.5, pp. 12–13). The average moving speed during tamping was 2.7 mph, with a maximum speed of 12 mph (moving to work location). The vibrations and shocks in the horizontal and vertical axis are explained by the forces and shocks from the tamping of ties and rail track, rhythmic machine travels, repetitive quick forward accelerations, braking maneuvers and the excessive wobble of the seat (loose/worn joints/bearing).
Table 1. Vibration measurement results of the operator seat system in a tamper.
The results of the WBV measurements on the seat pad showed basic rms aw values of 0.23, 0.05, and 0.35 (m/s2) for the x-, y- and z-axis respectively, and a vector sum (m/s2) of 0.48 for the entire measurement period (2 h) and all tamping work processes. The SEAT vector sum value av on average shows a SEAT of 0.94, indicating a slight reduction in exposure compared to floor measurements. The primary vibration exposure appears to come from the fore-after (x) and vertical axis (z), and lateral vibration values were comparatively low. Overall, the vibration attenuation performance of the seat was inconsistent, showing reduced and increased vibration transfer (SEAT) values for all axes. Some of the Crest Factors (CFs) in the forward (x), lateral (y) and vertical (z) directions reached or exceeded the critical value of >9 given by ISO 2631-1:1997. The vibration dose ratio (VDV/awT¼) was not exceeded. The basic rms vibration values for each horizontal x-, y-, and vertical z-axis, the vector sum results av, the Crest factor (CF), and the MTVV and VDV/awT¼ values are listed in the table for the overall measured time (t = 2 h). Selected work cycle segments (each t = 15 min) with the highest SEAT or vector sum av results are also listed below (see Table 1).
Importantly, as shown in Figure 5 and Figure 6, a rubber buffer located in the lower part of the seat spring system designed to control and stop excessive downward movement of the seat was broken and defective (arrow), and more likely than not renders the suspension system ineffective and unsafe in absorbing high input vibrations and shocks.
Figure 5. Detail of rear lower seat system; bellows removed. Downwards rubber buffer broken (arrow). On the right: showing the security upwards stopper belt (© E. Johanning).
Figure 6. Lower part of operator seat spring system showing suspension system and end-stop rubber buffer (1) (source: Repair Manual GRAMMER Seat Spring System MSG85—February 2004).

4. Discussion

Overall, the vibration measured on the seat pad of the machine operator seat was similar and typical compared to other maintenance-of-way equipment measurements [13]. However, a similar published study of a modified suspension seat design or occupational sentinel event study is not known. This case report is unique and novel, and there are no other similar studies published in the online literature utilizing search engines such as PubMed, Google Scholar, Web-of-Science, Reference manager Endnote 2025 or Google search. The steps involved in the investigation and prevention of occupational sentinel health events include identification and response with a thorough root-cause analysis, identification of contributing factors, underlying causes and active failures at the workplace, and development of corrective actions. Prevention strategies include the implementation of the hierarchy of controls (elimination, substitution, engineering controls, administrative controls, and personal protective equipment), such as in high biomechanical risk of construction and rail infra-structure workers [14]. Based on these results of an industrial hygiene site investigation, tamper operators should be monitored, and suitable exposure controls ought to be implemented. These include prevention measures such as medical surveillance, worker education, exposure time management and technical interventions such as undercarriage damping, suspension cabs and suitable seat suspension [15,16]. Early air-sprung suspension seats often amplified low-level chassis vibrations because their damping does not suit the vehicle’s air suspension tuning. Adjustable seating enhances operator comfort and facilitates complex machine operation by mitigating low-frequency vibrations and sudden impacts; however, optimal performance depends on appropriate seat design, correct user adjustments, and regular maintenance. Nowadays, highly specialized seat manufacturers can provide customized seats that can reduce vibration transmission to the lowest level that can reasonably achieved. Active, passive, and semi-active seat suspension systems reduce vibrations transmitted to vehicle occupants. Lowering body displacement and acceleration reduces WBV exposure to the operator and may improve ride comfort [17]. Good SEAT values in the vertical z-axis of 0.5, representing a 50% reduction, have been reported for Grammer Mfg. (series MSG95) seats without any attachments under laboratory conditions (personal communication). Poor selection of seat suspension systems can easily result in a higher vibration exposure than necessary as in this sentinel health event investigation [18,19]. The seat suspension system must also be selected so that, in typical use, it is unlikely to hit its top or bottom end-stops. Striking the end-stops creates shock vibrations, increasing the risk of back injury. Modern suspension seats require the operator’s individual weight and height to be adjusted with the suspension seat handles provide optimal suspension functioning. However, due to the additional weight from the control consoles and monitors sitting on top of the mechanical system, as well as the limited operator field of view ahead, requiring a low seat height, the necessary suspension range was likely not achieved. As a result, the swing system was not able to absorb the impacts, and hit the bottom end-stopper so hard that the rubber cushion broke.
All seat suspension systems have a range of frequencies that they amplify. If the dominant frequencies of the vehicle vibration fall within this amplification range, the seat suspension will make the driver’s vibration exposure worse [13]. ISO 10326-1:2016 [11] covers general seat testing, while ISO EN 7096:2000 (earthmoving) [20], ISO EN 5007 (tractor) [21], and EN 13490:2001 [22] specify performance criteria for utility vehicle seats. ISO 10326-2 (2022) [23] addresses laboratory evaluation for railway vehicles. However, under field conditions the performance of suspension seats may greatly differ from laboratory settings, and active suspension seats do not necessarily provide superior performance [3,24,25,26,27,28,29]. In an earlier study of a tamper machine (Harsco Rail Mfg. (Harsco Rail, Charlotte, NC, USA), Switch Tamper 6700 (ATS 9605)), the vibration exposure levels were slightly higher (vector sum av 0.82, z = axis SEAT value 0.91). The power spectral density (PSD) analysis of this Harsco tamper suspension seat indicated that the operator experienced vertical (z-axis) exposure to SEAT levels up to five times higher for frequencies below 10 Hz, coinciding with the human lumbar spine resonance range of 4 to 8 Hz (see Figure 7) [13].
Figure 7. Example of Power Spectral Density (PSD) and seat transfer function (SEAT) in the vertical z-axis for a comparison tamper vehicle (Harsco Switch Tamper 6700 (ATS 9605)) [13].
During the worst-case tamping scenario involving WBV exposure, the action limit of 0.5 m/s2 set by the European Directive on minimum health and safety requirements was surpassed based on the vector sum result [16]. However, in the US there are no laws setting numeric values and regulatory occupational health agencies have advocated to keep occupational exposures as low as technically feasible under the General Duty Clause [30,31]. In this sentinel case investigation, the modified seat spring system of the tamper had several shortcomings. The additional attachments (control console on right and left side, several attached monitors) added considerable additional weight to the driver’s weight and likely caused the seat suspension system to bottom out, which in turn caused the rubber end-stops to break and exposed the operator to added shocks. The seat had excessive wobble and was not securely mounted and braced as required by the US Federal Railroad Act (FRA 49 CFR Ch. II (10–1–23 Edition) § 229.119 Cabs, floors, and passageways). The workers explained that the tamper vehicle is transported on railcars to the working locations and that the railcar vibration and handling would cause equipment inside the cab to be tossed around and bounced during transport. The added side consoles increase stress on seat joints and bearings. Engineering best practice is to mount heavy consoles and monitors separately from the seat suspension, as their weight restricts seat adjustment and adds unnecessary load.
The claimant, a longtime tamper operator, had reported at age 52 years debilitating neck and lower back disorders (spondylosis, discogenic pain, lumbar radiculopathy) consistent with the disorders shown in epidemiological and medical studies of WBV-exposed workers [32,33,34,35,36,37,38]. He was working for 26 years as a maintenance-of-way worker operating various specialized rail vehicles and tools [39,40,41,42,43,44]. He worked typically 8 to 12 h shifts four days per week, plus overtime. He had a body weight of 90+ kg, and with that the tolerable weight tolerance of the seat was probably exceeded. Based on the overall findings and evidence it was concluded that the defective seat system was a contributing causal factor as defined by FELA. The case was settled out of court prior to this publication.

5. Study Limitations

This investigation examined a single machine with a defective aftermarket seat, using vibration measurements from a track section chosen by railroad representatives, which limits its generalizability. These findings may not necessarily reflect other tampers or suspension seats used in the rail industry, especially when proper inspection, repairs and maintenance are performed. Currently, there are no published reports of comparable situations or baseline industry data for comparison from online sources. It has been suggested that train seat’s SEAT value can be determined either by measuring it through simulated tri-axial train vibrations in a laboratory or by calculating it using vibration data collected from an actual railroad vehicle combined with laboratory measurements of seat transmissibility using random vibrations [43].
Due to logistical constraints, we could not conduct a PSD analysis to identify dominant vehicle frequencies at this seat mount system; such data would aid seat engineering design and damper selection. Previous measurements on a HARSCO tamper, including PSD analyses, may offer useful reference points (see Figure 7). The SEAT values for this vehicle and suspension seat were 1.08 (x-axis), 1.22 (y-axis), and 0.91 (z-axis).
The operator’s responsibilities are both varied and challenging; they must use their hands and feet to work the controls, keep an eye on the monitors, oversee the tamper as it adjusts the ties up close, and pay attention to the laser guidance system ahead and other crew members along the track. These demands cause the operator to frequently shift their upper body and head, often leaning forward and losing contact with neck and upper back supports. The height, body weight and experience/skill of other tamper operators as well as differences in track structural build may vary considerably and may modify vibration results.

6. Conclusions

The tested suspension seat appeared to underperform and was overloaded with the aftermarket control devices, attachments and modifications. The suspension system’s end-stopper was damaged. The seat system had excessive play and wobbles; it was not firmly braced and attached as required by FRA regulations. The results of the WBV measurements on the seat pad showed basic rms aw values of 0.23, 0.05, and 0.35 (m/s2) for the x-, y- and z-axis respectively, and a vector sum (m/s2) of 0.48 for the entire measurement period (2 h) and all tamping work processes. The SEAT vector sum value on average shows a SEAT av of 0.94, indicating a slight reduction in exposure compared to floor measurements. The vector sum (av) results ranged from 0.26 m/s2 (no tamping) to a maximal 0.55 m/s2 (tamping). The seat transfer (SEAT) analysis showed magnification of vibration input and variable performance of the suspension depending on operational tasks. The primary vibration exposure appears to come from the fore-after (x) and vertical axis (z), and lateral vibration values were comparatively low. The vibration attenuation performance of the seat was inconsistent, showing reduced and increased vibration transfer (SEAT) values for all axes depending on tamping tasks and accelerations values. This investigation found that the tamper’s modified seat spring system was faulty and unsafe. The aftermarket attachments increased the load, causing the suspension to bottom out and break rubber end-stops, exposing the operator to more preventable vibration and shocks. The seat also wobbled excessively and was insecurely mounted, issues that could be fixed with proper maintenance. Mounting heavy consoles and monitors apart from the seat suspension avoids limiting seat adjustment and likely improves the SEAT performance. Effective collaboration among vehicle users and seat and vehicle engineers can reduce WBV exposure risk and improve safety and efficiency.

Supplementary Materials

The following supporting information can be downloaded at: https://www.youtube.com/watch?v=jMVVB3xcHI0 (accessed on 4 February 2026), Video S1: Tamper Seating Investigation.

Funding

This research received no external funding.

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Material. Further inquiries can be directed to the corresponding author.

Acknowledgments

I want to thank Benno Goeres, Head of Seat Test Laboratory (retired), formerly BIA/DGUV, St. Augustin, German, for his valuable input.

Conflicts of Interest

The author declares no conflicts of interest. The author E.J. is a US member of the ISO/TC 4 appointed by the ISO Member Body ANSI (United States). E.J. has been evaluating and treating workers with WBV exposure and represented some in disability claims. He follows the Ethical Guidelines of the International Committee of Occupational Health (ICOH), American College of Occupational and Environmental Medicine (ACOEM), and the Association of Occupational and Environmental Clinics (AOEC) (all available online). This FELA claim case was settled out of court prior to this publication.

Abbreviations

The following abbreviations are used in this manuscript:
ANSIAmerican National Standards Institute
CFCrest Factor
FELAFederal Employer Liability Act
FRAFederal Railroad Act (or Administration)
ISOInternational Standard Organization
MTVVMaximal Transient Vibration Value
SEATSeat Effective Amplitude Transmissibility
VDVVibration Dose Value
WBVWhole-body vibration

Appendix A

Basic method to characterize vibration (ISO 2631-1:1997)
a w = 1 T ∫ 0 T a w 2 ( t ) dt
Crest Factor calculation (ISO 2631-1:1997)
CF = max     a w P V ( t ) a w
Critical ratio values (ISO 2631-1:1997)
M T V V a w = 1.5   VDV a w ⋅   T 1 / 4 = 1.75

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Editorial for the Special Issue of Vibration: Nonlinear Vibration of Mechanical Systems
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