D0605


SELF-SECURING RAIL SPIKE


Moishe Garfinkle PhD
Advanced Railway Technology Group
Philadelphia PA


ABSTRACT

Regardless of the rail securing means, whether spike, bolt or myriad of complex securing means that have been developed, localized tie splitting tends to decrease the grip of the securing means to the tie, and if severe enough, loosen the spike or bolt beyond the point of safety. In response to these deficiencies, a unique Self-Securing Rail (SSR) spike is disclosed whose grip on the tie is not affected by seasonal variations in ambient temperature and moisture, and consequently not affected tie swelling or shrinking. Moreover the grip of the SSR Rail spike is not loosened by tie splitting.

Althought the SSR spike appears to resemble the conventional hairpin spike, the mechanical resemblence is superficial.

Unlike all other rail securing means wherein the wood tie grips the spike, the SSR spike grips the wood tie. Accordingly the SSR spike alone grips the tie by loading the wood grain in compression rather than customarily in tension. The effect of this compression loading is to secure the spike and maintain the spike grip independently of temperature, moisture content or even cracks in the cross-tie.

Because the prongs of the SSR spike are tightly secured, they are not subject to low cycle fatigue, a principle cause of failure of hairpin spikes.



1    BACKGROUND

The stability of railway track work, particularly when laid on wooden cross ties, is a constant concern of railroads. The grip of the spikes or bolts or other securing means which holds the rails to the tie plates and the tie plates to the cross ties depends on the resiliency of the wooden ties to maintain the grip. This grip is adversely affected however both by the vibrational loads imposed by passing trains, particularly heavy freight operation, and by seasonal variations in ambient temperature and moisture which causes ties to swell or shrink, greatly affecting the resilience of wood, and which, if sufficiently severe, can result in splitting of the cross ties.

In addition, the increasing employment of continuous-welded rail (CWR) further accentuates this problem inasmuch as lengthwise thermal expansion of the rails is suppressed by rigidly securing the rails to the tie plates which are in turn rigidly secured to the ties, with the ties immobilized by the ballast. Obviously any relative motion of any of the components imposed between the rails and the ballast can destabilize the Track work and widen the gauge, which generally occurs when the grip of the wooden ties on the spikes or bolts is compromised.

Essentially CWR Track work is unstable and consequently the safety of passing trains depends on continual inspection and retightening or replacing spikes or bolts to keep the rails secured to the tie plates and the tie plates secured to the ties. Otherwise relative motion between the tieplate and tie can occur, resulting in either lose of spike, gauge widening, or both. Although there are perhaps 15,000 or more bolts or spikes per mile of track that must be inspected, retightened or replaced to maintain CWR stability, nevertheless the aggregate initial and ongoing costs of CWR track is still less than than that of separate section track, indicating primarily the speed and economics of automatic track laying. Evidently a means to more permanently secure the tie plates to the ties would go far in significantly improving the economics of CWR track, particularly in regard to spike loosening and gauge widening. This concern is particularly relevant in regard to high-speed rail where safety issues, even more than economic issues, are paramount, and these two issues must be satisfactorily reconciled.

2   PRIOR ART

Figure 2.1 show three common forms of rail spikes. What they have in common is that they all impose a tensile load on the wood grain that tends to open the grain. Hence the rely on the resiliency of the wood to maintain the grip. However vibration, temperature and moisture have the effect of further opening the wood grain, finally initiating cracks in the cross tie and causing loss of the spike grip, whether cut, hairpin of screw. Accordingly many variations on spikes and bolts have been proposed to alleviate this maintenance problem by including components that essentially lock the spike to the cross tie.   What such devices have in common is that they all impose a tensile load on the cross tie, relying on the resiliency of the wood to maintain the grip between the spike or bolt and the tie.

Figure 2.1 Compression loading of Cross Ties by Common Rail Spikes

The ordinary cut spike on being driven severs the polycellulose fibers of the wood tie. The cut spike relies therefore on the resiliency of the wood to provide a strong compressive load on the spike in the gauge direction. Hence the spike as shown in Figure 2.1 is secured by friction alone. Normal variations in moisture content of ties however results in the friction load continuously varying. Under either high-speed passenger or heavy freight operations this friction load has generally proven inadequate to reliably retain the spike under the vibratory loads to which the spike is subject, requiring continual inspection and resetting of the spikes.

The hairpin spike is so constructed that on being driven into the tie plate the prongs of the spike are forced apart laterally. The wood grain compressively resisting this lateral displacement, ostensibly wedges the hairpin spike into the tie plate. Normal variations in moisture content of the ties however results in a continuously varying compressive load on the prongs, alternatively loosening and tightening the hairpin spike wedged in the tie plate. The consequence is fretting damage to the portion of the spike wedged in the tie plate under the vibratory loads to which the spike is subject, resulting in possible fatigue failure of the spike.

The screw spike ostensibly should provide the strongest hold between the tie plate and tie. The wood fibers, on being displaced by the screw, become wedged between the threads which behave as teeth, resisting withdrawal. The extent of wedging however depend on the resiliency of the wood which is dependent on the normal variations in moisture content of the ties. Moreover, lubricated by moisture the screw spike tends to unscrew under vibratory loading, requiring retightening of the spike on a regular schedule.

Stafford proposes an otherwise ordinary spike which effects an interference fit between the spike and the tie plate to essentially jam the spike into the tie plate hole when the spike is driven. However the Stafford spike does not enhance the grip between the spike and the tie.

Walker proposes an otherwise ordinary spike with a prong which curves into the tie as the spike is driven. However the Walker spike does not secure the tie plate from movement in the gauge direction as the grip is along the grain.

Schiro proposes an otherwise ordinary spike with a centrally bored hole which holds a locking pin. Once the spike is driven the locking pin is inserted and upon driving the pin toothed engaging members penetrate the wood to lock the spike in place. However the Schiro spike would not only be considerably more expensive than ordinary spikes but would require and additional operation to secure the spike.

Taylor-Smith proposes a special spike comprising several elements which jam together to lock the rail to the tie plate by means of a contact plate. However the Taylor-Smith spike would not only be considerably more expensive than ordinary spikes and probably could not be set by automatic track laying equipment without major modifications.

Conventional hairpin spikes, often used in light-rail service, rely in an expanded section or bulge that compresses as the spike is driven into the tie plate, imposing a tensile load which grips the wood grain of the tie. Such tensile loading on wood is compromised however by changes in temperature and particularly moisture content of the tie, which progressively loosens the grip.

Subsequently vibrational loading excited by passing trains causes relative motion between the hairpin spike and the tie plate, resulting in fretting of the spike. Such damage can often lead to failure of the spike, often by fatigue. In contrast cut spikes simply loosen under such vibratory loading. With freight trains running heavier and passenger trains faster, this problem of insecure spikes under high vibratory loading can only exacerbate.

To eliminate this problem of loosened tie plates and gauge widening a unique steel Self-Securing Rail (SSR) spike is disclosed that would be particularly advantageous compared to cut spikes for high-speed passenger and heavy freight operation, as well as a reliable replacement for hairpin and screw spikes. The grip of the SSR Spike neither depends on the condition of the ties at the time the spike is set nor during the usable life of the tie. Moreover the SSR Spike is economical to fabricate and is amenable to conventional automatic track laying equipment while adhering to acceptable Track work standards.

3   SELF-SECURING RAIL SPIKE

Rather than relying on the resiliency of the wood ties to maintain a grip on the spikes as does conventional spikes, the SSR Spike relies on the elasticity of the spike prongs to maintain a clamping effect on the wooden ties independent of shrinkage or expansion of the tie due to changing moisture content. Moreover, because the grip of the SSR Spike is compressive, even if the tie severely splits in service the grip of the barbs on the SSR Spike prongs will not be compromised.

The dimensions of the SSR Spike is essentially the same as the conventional hairpin spike as shown in Figure 3.1.  

Figure 3.1 Dimensions of Self-Securing Rail Spike

 Although superficially resembling a hairpin spike, the SSR Spike is conceptually not a hairpin spike and should not be confused with a hairpin spike. As seen if Figure 3.2 the body of SSR Spike 1 is fabricated from steel plate with teeth 2 either cut or impressed into the sheet surface and configured to facilitate driving of the spike, with the plate beveled to chisel-edged ramps 3 before the plate is bent to form the loop 4 and the prongs 5: a simple, economical forming process.

Figure 3.2 Construction of Self-Securing Rail Spike

4   SPIKE RETENTION

Wood is a composite material comprising strong polycellulose fibers in a relatively weak lignin matrix with the fibers roughly parallel to the observed wood grain. Accordingly wood ties are strongest in tension in the direction of the strong fibers and weak perpendicular to the fibers, explaining why ties preferentially split along the grain in the gauge direction. Setting any fastener into a wood tie, whether spike or screw, imposes a tensile load on the grain adjacent to the fastener perpendicular to the wood fibers, the direction of lowest strength, which initiates a localized split in the tie along the grain. If this localized split elongates the fastener can loosen, compromising the track gauge.

Common woods can absorb water up to 30 to 35 percent of their dry weight, and the hardness of wood, which directly relates to its ability to grip a spike, varies inversely with its moisture content. At a typical moisture content of 12% white oak is three times are hard are white pine, which indicates the great span of hardnesses encountered with typical woods that might be used for ties without even considering different moisture contents. Understandably in many areas less desirable woods must be used in track work because they are readily available and hence economical. Moreover, common spikes are not designed for any particular wood or climatic conditions: whether the wood is generally hard or soft, or saturated or desiccated.

Setting any fastener into a wood tie, whether spike or screw, imposes a tensile load on the grain adjacent to the fastener perpendicular to the wood fibers, the direction of lowest strength, which initiates a localized split in the tie along the grain. If this localized split elongates then the fastener can loosen, compromising the track gauge. For example, the sequence of spike setting is compared below for the hairpin and SSR spikes, showing the loads imposed on the wood grain.

Figure 4.1a Spikes Aligned for Setting

Both spikes are aligned identically for setting.   When partially set however the SSR spike begins to grip the cross tie by compressing the wood grain. The chisel-edged prongs slip between the wood fibers rather than cut across them.

Figure 4.1b Spikes Partially Set

When fully set the hairpin spike imposes a strong tensile stress on the wood grain which tends to open the wood grain. In contrast the SSR spike imposes a compression load which tends to close the wood grain.

Figure 4.1c Spikes fully Set

Hence, the SSR spike not only opposes cross-tie cracking but opposes elongation of pre-existing cracks. Essentially, the wood between the prongs of the SSR spike as shown in Figure 4.2 is under compression. This compression loading tends to close existing cracks in the cross tie.

Figure 4.2 Compressive Loading of Cross Tie

Equally importantly, the polycellulose fibers wedged between the prongs are in the gauge direction and are intact rather than severed. Consequently the fibers in the wedged area remain continuous, securing the spike to the cross tie. This SSR Spike orientation is particularly favorable in resisting lateral displacement of the rails and thereby maintaining gauge, an ongoing concern along curved sections serving high-speed passenger and heavy freight operations.

Figure 4.3 SSR Spikes Installed in Tie Plate

5   RETENTION IN SPLIT TIES

The splitting of wood by alternate saturation and desiccation is a common occurrence, particularly with expansion of trapped water during freezing. All of the spikes discussed, with the notable exception of the SSR Spike, lose a substantial portion of their grip on the tie when the spike is intersected by a split. The cut spike loses essentially all of its grip in a split tie while the hairpin spike lose enough to compromise its grip on the tie plate, leading to fretting damage and possible fatigue failure.

The screw spike, dependent on the resiliency of the wood tie to maintain the wood fibers wedged between its threads, will simply loosen. In contrast, as the immobility of the SSR Spike arises from elasticity of the steel spike rather than the resiliency of the wood tie, tie splitting has a minimal effect on SSR Spike retention. Any tendency of the SSR Spike to displace upwards under vibratory loading will further jam the compressed polycellulose fibers between the prongs, resisting further displacement. Moreover the clamping of the wood wedged between the prongs will impose a strong compressive load on splits as shown in Figure 4.2, minimizing their growth. Hence the SSR spike maintains firm contact with tie plate, averting fretting damage and low cycle fatigue

6   RAMP CONFIGURATION

Consider now the SSR Spike configuration. The compressive load imposed to embed the teeth into the wood fibers depends on how far the prongs are forced apart. This spread depends in turn on both the ramp configuration at the end of the prongs and the hardness of the wood ties. For regions where very hard timber is readily available the spread can be minimal, as shown in Figure 6.1a, with the ramp configured as shown. Hence the SSR Spike can be readily driven, yet positively locks in place.

Where only softer woods are available, a wider spread is required as shown in Figure 6.1c, but the driving force would only marginally less from that required for the harder woods because a greater driving force is required for a greater spread.

Figure 6.1 Control of Compressive Loading

For both ramp configurations the compressive stress imposed on the wood fibers will resist splitting, regardless of moisture content. Hence the SSR Spike can be readily configured for the diverse woods used for ties, particular where more desirable timber might not be available.

7   FATIGUE FAILURE

Low-cycle fatigue is a recurring problem with common rail spikes. Unless the spikes are essentially jammed into the appropriate holes of the rail baseplate then relative motion between the rail baseplate and the spike can result in fretting damage that can lead to fatigue cracking and eventual failure, often within the tie and therefore out of sight.

Figure 7.1 Control of Compressive Loading

The SSR spike is not susceptible to such relative motion as the prongs are jammed against the rail baseplate on being driven apart on installation, thereby creating essentially an interference fit.

8   TRACK WORK

Track work is not forever and spike removal is a necessary operation in track work when ties or rails have to be replaced. However the very compressive forces that secure SSR Spikes ostensibly render their removal most difficult. A forced removal would further wedge wood fibers between the prongs, probably tearing a portion of the tie.

The great difficulty in removing SSR Spikes from ties can complicate trackwork. Essentially the solution involves the relative cost of the SSR Spike compared to the aggregate cost of the trackwork. Generally trackwork involves replacing deteriorated ties with new ties. The most economical means of tie replacement is to slice off the head of the SSR Spike flush with the tie plate, lift the rails and knock loose the tie for disposal, prongs and all.

9   CONCLUSION

The grip of the SSR Spike disclosed herein, dependent on the elasticity of the steel spikes rather than the resiliency of wood ties, does not diminish with seasonal variations in ambient temperature or humidity. Moreover the SSR Spike


Moishe Garfinkle PhD
Advanced Railway Technology Group
Philadelphia PA
(215) 235-5042


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