Patent Application
20200038737
Not Applicable
Not Applicable
Not Applicable
The present invention relates to low-profile endless track mechanisms for the purpose of gliding over rough or soft ground. It especially relates to the use of such mechanisms to ski or skate without the need for snow or ice.
For a mountain bike to roll over obstacles such as rocks, roots and small branches, as encountered on a typical trail, the minimum wheel diameter is approximately 2 feet. Mounting one such wheel per leg has been tried, e.g. the “Chariot Wheel”, but leads to poor fore-aft stability and usually requires a second smaller wheel. Wheels up to about 8″ diameter have been mounted under a skate boot, e.g. the “Rollerblade Coyote”, but the small diameter rolls poorly over obstacles, and the high ground contact pressure leads to poor rolling resistance on soft ground. Larger wheels under the boot result in excessive lateral torque on the knee, and large wheels fore and aft of the boot cause excessive weight and poor ground clearance.
Compared with wheels, the use of endless belts or tracks allows much larger contact area and lower ground pressure. This translates to less work done to indent the ground, and the potential for lower rolling resistance. Tracks can also provide a more gently curved or ramped bottom surface to roll over obstacles more smoothly.
The patent literature includes many examples of tracked vehicles as used on heavy machinery, snowmobiles, military vehicles (tanks). There are relatively fewer examples of tracked skates and grass-skis.
The skate/ski prior art falls into four main groups: 1) belts or tracks sliding on an oval track (e.g. Pierce U.S. Pat. No. 342,458, 1886), 2) belts or tracks rolling on an oval track using recirculating balls or rollers (e.g. Fohr U.S. Pat. No. 675,824A, 1901, Chevreau, U.S. Pat. No. 1,508,218, 1924, Abdulaev, U.S. Pat. No. 7,976,064B2, 2011), 3) belts or tracks with attached rollers engaging an oval or indented oval track (e.g. the Rollka grass skis), 4) belts or tracks suspended by rollers (e.g. Bierly, U.S. Pat. No. 1,583,114, 1926, and Rieske, U.S. Pat. No. 2,412,290, 1946), and 5) non-inverting belts or tracks suspended by rollers (e.g. Miller, U.S. Pat. No. 889,946, 1908 and Freilich, U.S. Pat. No. 5,580,096, 1996).
The first group tends to have very high friction due to the sliding interface.
The second group typically uses a continuous belt and small closely spaced rollers or balls. The belt needs to be quite thin and soft to be able to make the small radius turn at either end of the skate, and this runs counter to the need for bending stiffness to spread the load on the ground and to have low rolling resistance against the balls or rollers. This group is very sensitive to dirt fouling the mechanism due to the small size of the rolling elements, and their close proximity to the ground. Putting full weight on a small obstacle like a root or rock is likely to damage the rolling interface.
The third category includes the most successful type of grass-ski, first manufactured by Rollka. This device has acceptably low friction for downhill skiing but not for cross-country. It is inherently fragile for its weight and requires a soft ground surface, i.e. grass. The widely-spaced roller assemblies result in high local ground pressure, with sections of unsupported belt. Going over a branch or root can indent the belt and cause excessive tension that damages the track/roller mechanism.
The fourth category is the standard for modern ground-moving machinery, snow-cat's, snowmobiles and tanks. In the skate examples given (Bierly and Rieske), the rollers are large and widely spaced, and the resultant ground contact pressure is not much different than if the track were not there. Use of a stiffer belt or very high belt tension is not practical due to excessive friction and hysteresis. Use of small, closely spaced rollers and hinged links (e.g. Miller), is advantageous, but there is a typically a weak point at either end of the track due to the need for large end-rollers.
The fifth category has hinged links that can bend around the end-rollers, but resist being bent in the opposite direction. This allows the track to maintain its shape even when the rollers are large and widely spaced, but at the expense of very high tensile loads in the hinge joints. As described by Miller and Freilich, the hinge-stop mechanisms represent pinch-points for dirt and gravel. This causes extreme loads on the hinges leading to premature failure.
There remains a need for a low-profile, lightweight skate runner with very low rolling resistance that can handle terrain with roots, rocks, and soft ground, is reasonably safe, does not get fouled by dirt and detritus, and can be manufactured cheaply.
The present invention is a tracked skate runner appropriate for mounting to a skate boot as well as other uses. It has a roller assembly with a skate frame and rollers which bear against an endless track of hinged links. The frame provides a channel with sidewalls to shield out dirt, and a top shield to protect the rollers from dirt which shakes loose from the top recirculating portion of the track. The track is substantially devoid of pinch-points that can get jammed with ground debris and cause a kink in the bottom load-bearing portion of the track.
In various preferred embodiments, the following features may be included in various combinations: The skate frame may be a structural channel made from an aluminum extrusion or bent sheet metal. The links may have flanges defining a guide channel for the rollers, and the rollers may extend below the sidewalls as a safety feature in case the track breaks. The end-rollers may be spool-shaped and overlap the adjacent rollers to shorten the span and better support the track. The links may have knuckles which mate to form hinges that provide a smooth interdigitated rolling surface for the rollers. The hinges may have internal hinge-stops to prevent reverse bending without presenting external pinch-points. These stops may employ bumps on knuckles engaging webs between knuckles and visa-versa to form a strong “handshake”. The rolling surfaces of the rollers and links may be made of hard materials to provide low rolling resistance, and the ground-contacting sides of the links may have elastomeric treads to protect the hinges and provide a smooth ride. The treads may be chamfered to avoid pinching ground debris. The skate frame may have brackets to connect to a boot, and the dimensions of the tracked skate runner may be tuned for the skate application. The bottom portion of the track may be rockered 0.15 to 0.6 inches, the pitch of the track may be 0.5 to 1.2 inches, the end-rollers may be sized such that the angle between links is 120-144° (equivalent to a 6-10 sided polygon), the width of the links may be 0.7-2 inches, and the thickness of the treads may be 0.1-0.5 inch.
Using the above features and dimensions can minimize stresses on the links which enables the use of polymer rollers and link-bodies which can be manufactured cheaply by injection molding, casting or 3D printing.
For the skate application, the preferred thickness “t” of the treads is 0.1 to 0.5 inch, with 0.3 inch being especially preferred. The treads are preferably chamfered or rounded to avoid pinching debris at the bottom/front of the skate. This reduces rolling resistance and helps avoid delamination of the treads. The included angle “ϕ” between treads on the bottom surface of the track should be at least 30°, and is preferably 45°. This largely avoids pinching while maintaining a large tread contact area.
The preferred track pitch is between 0.5 and 1.2 inches, and a pitch of 0.8 inches is especially preferred. Making the pitch too small reduces the surface area of the treads (for a given tread thickness and chamfer), and drives up the parts count which increases cost. Making the pitch too large results in large end-rollers that increase the height of the boot off the ground and cause excessive lateral torque on the knee.
Tracked skate runner 20,
The curvature need not be exactly constant, but there are advantages to having a smooth curve. Angles between adjacent links are minimized, presenting a smoother surface for the rollers to roll over. Avoiding large changes in curvature avoids expansion and contraction between treads that would cause rubbing on the ground, and it avoids regions of small curvature that apply excessive ground pressure. The preferred radius of curvature (50 inches) also helps keep the track from sagging down from the roller assembly when the skate runner is lifted off the ground.
The
The end-rollers are preferably sized such that the links engaging them form an included angle between 120 and 144°. This corresponds to a polygon with 6 to 10 sides. The end-rollers can be round, but polygonal is preferred to increase contact area and reduce wear. For a given track pitch, smaller end-rollers result in greater energy lost to impact as the links engage the rollers. Smaller end-rollers also result in greater noise due to height oscillation of the top section of track. Large end-rollers result in greater height of the roller assembly which leads to excessive lateral torque on the knee. Octagonal end-rollers are a good compromise. These result in an included angle of 135° of the engaging links.
In other embodiments, the channel could be made from an assembly of parts, for instance two side plates separated by standoffs, and the top dirt shield need not be structural.
Referring back to
A typical solution is to use the tracks only on soft ground, or to add suspension or a soft tread to the rollers. The preferred solution described herein is to add compliance to the outside of the track. This is the reason for treads 36 attached to the link-bodies 34. To understand the advantage, consider terrain with hard-packed dirt and a single small pebble. Using compliant rollers, each roller deflects as the track goes over the pebble, and energy is absorbed (and partially returned) many times. With compliance on the outside of the track, the tread in contact with the pebble sees only one compression/extension cycle. This translates to less energy loss and lower rolling friction.
The preferred material for the rollers and link bodies is a hard, resilient, high-strength, low-friction, high-wear polymer such as Delrin, Nylon, or one of the hardest grades of urethane. Many other polymers would also be acceptable, and various fillers such as glass-fiber, carbon fiber, PTFE, graphite and moly-disulphide may be beneficial to increase strength or improve lubricity. Using polymers, the initial (low-strain) Young's modulus is preferably greater than 20 ksi, to avoid excessive deflection, and less than 600 ksi to avoid excessive contact stress. If 3D printed, Nylon alloy 910 is a good choice. The rollers and link bodies need not be made of the same material, and it is acceptable to use ball bearings rolling directly on the link bodies. Methods of fabricating the link-bodies and rollers include injection molding, 3D printing, machining and casting.
While polymers are preferred to minimize cost, there may be a performance benefit in strength/weight ratio and rolling resistance to using metal links and/or rollers. Fabrication methods include machining, metal injection molding, casting and 3D printing.
For the skate application, treads 36 in
The rollers and link bodies have mating male and female features configured to resist side loads on the track, while minimizing friction. Example features are a raised ridge on the rollers engaging a groove in the track, or a raised ridge on the links engaging a groove in the rollers. The preferred method, as shown in
As shown in
When traversing an obstacle such as a root or rock, it is desirable to prevent the hinges from deflecting inward (reverse bending), since this forms a kink in the track that the rollers must roll over and results in high rolling resistance. The normal approach is to use an external hinge-stop as per Miller or Freilich, however, this results in a pinching action as the track comes around the front end-roller and flattens out. If a small rock or sand gets pinched, it is likely to damage the hinge-stop, as well as causing very high loads on the hinge pin and knuckles.
The preferred solution, as shown in
Note that the bumps on the knuckles of one link engage the webs between the knuckles of the adjacent link and visa-versa. This hand-shaking effect doubles the strength of the hinge stops. Also important is that the knuckles are closely interdigitated and the webs can be supported on three sides. This maximizes strength, and is an enabling feature for allowing the link-bodies to be made of plastic. For best results, the links should have at least three knuckles on each end. Use of four knuckles on one end and five on the other is especially preferred.
In
While pinch-points between link-bodies are to be avoided, small pinch-points between treads are acceptable as long as they allow the bottom portion of the track, under load, to assume the normal curvature defined by the mid-rollers. For instance, edges 37 of treads 36 may overhang the bottom surfaces of the link-bodies. This results in a very small pinch point between treads which is beneficial to form a seal and reduce mud infiltration of the hinges. However, if debris gets pinched, the treads deflect enough to avoid a kink in the track that would overstress the hinge.
To prevent dirt intrusion from either side of the track, sidewalls 60 of skate frame 24 fit closely with link body 34. In this example, the sidewalls sandwich the link-body, extending over flanges 72 of link bodies 34 leaving small gaps 96. The mid-rollers 52 extend a small distance “d” below the sidewalls, preferably about 1/16 inch. In the event of the track breaking, the roller assembly will continue to roll forward leaving the track behind, and the rollers will touch down before the sidewalls. This reduces the likelihood that the rider will fall forward.
Some dirt may tend to stick to the sides of the links, especially if the ground is wet. As the track recirculates, the dirt will tend to fall downward and land on top dirt shield 58. Gaps 32 between the recirculating track and the dirt shield provide a “dirt window”, allowing the dirt to slide off to the sides. A slanted, tent-like or domed dirt shield may be used, but is not essential because the normal skating motion tends to bank the skate. Vertical vibration of the recirculating track also produces air currents which expel dirt. Any dirt that gets into the interior of the roller assembly will tend to deposit in the channel between the flanges 72 of link bodies 34. When the track recirculates, the links turn upside-down and the dirt falls on the dirt shield and migrates out the dirt windows. In this way, the skate track runner is self-cleaning.
In
The variations of the invention described above have certain desirable and surprising combinations. For instance, the use of flanged links enables the rollers to be mounted lower than the sidewalls (for safety), while still shielding out dirt, and it allows the use of spool-shaped end-rollers that overlap the adjacent narrower mid-rollers to better support the track.
Using plastic links, strength is a key concern. Having the rollers roll over the interdigitated knuckles of the links not only maximizes the amount of material available to make the hinge, but enables a hand-shaking hinge-stop devoid of external pinch-points that can get jammed with ground debris.
Hard rollers rolling on hard link-bodies provide very low rolling resistance but increase sensitivity to fouling, so the combination with sidewalls and a self-cleaning top-dirt shield is key. With hard rolling elements, the addition of soft treads to the links is important to maintain ride quality as well as to protect the link-bodies from damage from rocks and gravel. Adding the treads increases the risk of pinching debris, but this is mitigated by chamfering the treads.
For the skate application, there is a sweet spot of rocker, link width, tread thickness, track pitch, and end-roller link angles. Getting this combination right has eluded the prior art and is a major factor in the lack of commercial success of a tracked skate.
While the foregoing written description of the invention enables one of ordinary skill to make and use what is considered presently to be the best mode thereof, those of ordinary skill will understand and appreciate the existence of variations, combinations, and equivalents of the specific embodiment, method, and examples herein. The invention should therefore not be limited by the above described embodiment, method, and examples, but by all embodiments and methods within the scope and spirit of the invention.
