GB2050971A - Rotary crank assemblies - Google Patents

Abstract

A crank assembly for pedal bicycles or other apparatus comprises a crank lever formed in two parts 1, 2 the part 2 being caused to extend relatively to the part 1 during the working part of the rotation of the lever thus increasing the torque produced and then to retract so that lever is of a constant, shorter, length during the non-working part of its rotation. Extension and retraction of the part 2 may, as shown, be effected by a follower 7, 7a on the part 2 engaging a cam groove 8 in a fixed cam 9, or alternatively may be achieved by an eccentric mechanism. A fly wheel may be provided in the crank assembly. <IMAGE>

Description

SPECIFICATION A crank assembly This invention concerns improvernents in or relating to a crank assembly intended more particularly, although not exclusively, for use on manually, i.e.

foot-propelled, pedal cycles.

The primary object of the invention is the provision of a crank assembly that enables a greatertorque to be applied to the bottom bracket spindle and its chain wheel during the working or power stroke of the crank lever and that can be achieved with little or no extra effort by a person riding the dicycle and so improve the performance of the machine.

The limitations of the torque produced by a conventional pedal crank is its length which in turn is governed by the maximum distance from top dead centre to bottom dead centre, which a person can reasonably manage.

According to the present invention there is provided a crank assembly wherein each crank lever is adapted to be increased in length during its working or power stroke and is returned to a normal unextended constant length during its non-working stroke.

In the case of a bicycle, extension of the length of each crank lever during a power exerting movement is achieved between upper and lower dead centre of the foot pedal and the lever is fully extended while in its forward driving position and shortens to a normal unextended constant length before or on reaching bottom dead centre.

The variable length pedal crank of this invention overcomes the limitation above referred to by keeping the distance between top dead centre and bottom dead centre constant during a non-working stroke of the lever while extending the length of the crank lever on the power stroke.

The crank extends aftertop dead centre and is fully extended while the pedal is in the forward position. It shortens fully before reaching bottom dead centre.

According to a preferred embodiment of the invention extension and retraction of the crank lever is achieved and controlled by a fixed cam attached to the bottom bracket of a bicycle and which is engaged by cam followers on an arm connected to a part of the crank lever that is slidable in relation to a relatively non-slidable part that is secured to the bottom bracket spindle.

Although the invention is primarily intended to be applied to crank assemblies for foot propelled cycles it is to be appreciated that it may, with advantage, be applied to other machines or apparatus similarly powered by the use of pedals, e.g. crank actuated paddle boats whether foot or hand operated, invalid wheel chairs, children's toy vehicles and the like.

To enable the invention to be clearly understood and readily carried into effect a preferred embodiment thereof will now be described by way of example with reference to the accompanying purely diagrammatic drawings which illustrate the inventidn as applied to a pedal cycle and wherein: Figure lisa side view of the crank assembly, Figure 2 is a vertical section of a part of the assembly above a horizontal centre line and taken on the line il-ll of Figure 1, Figure 3 is a view of the outer extendible part of the crank lever, and Figure 4 is a side view of the assembled parts of the extendible crank lever.

Referring to said drawings the crank assembly comprises a pair of crank levers each consisting of two telescopic inner and outer main parts 1 and 2 respectively, the inner part 1 being connected to the bottom bracket spindle 3 and the outer part 2, which is slidable length-wise over the inner part 1, being provided with a pedal spindle 4.

The outer slidable part 2 is made in two halves (see Figure 2) that are bolted together at their outer ends by the attachment end 4c of the pedal spindle 4 that provides a taper fit, and at their inner ends by bolts 3 passing through lugs 6 on either side of the two halves of the outer part 2.

Movement of the outer slidable part 2 lengthways over the inner part 1 is effected and controlled by a cam follower 7, 7a that travels, when the crank lever is turned about the bottom bracket spindle 3, in a track or groove 8 (Figure 1) formed in a fixed cam disc 9 attached to the bottom bracket 10 of a bicycle.

The cam follower comprises two eccentric side by side bearings or rollers 7 and 7a (see Figure 2) and a stub 11 (see Figure 3), on which the cam bearing rollers 7 are mounted is formed with two different diameter eccentric portions that enable the eccentric cam rollers to be offset so that the roller 7 bears and rolls against the outer edge of the cam groove 8 and the other roller 7a rolls against the inner edge of said groove so as to provide a stable condition at all times without slackness or friction occurring during extension and retraction of the slidable part 2 of the crank lever. If only a single cam follower bearing was used there would be considerable friction when the bearing transferred from one cam face to the other because the bearing would have to reverse direction of rotation at the transfer points. The part 2 is formed with an oil way and air vent 2a.The approximate travel of the slidable part is indicated at T.

The crank assembly mechanism above described is of course provided at each side of a cycle.

On the left hand or near side of cycle (that is the side of the hub not fitted with a chain wheel) movement is transmitted from the cam follower 7, 7a to the slidable part 2 of the crank lever by a straight shaft (not illustrated) extending between the cam bearing stubs 11 and the outer part 2 of the crank lever.

On the right hand or offside of a cycle where the chain wheel and chain is fitted, i.e. as viewed in Figure 2, movement is transmitted from the inside to the outside of the chain wheel (not shown), which is located between the associated crank lever and a balance or flywheel 12 by an arm 13 disposed parallel to the plane of the stationary cam disc 9, and a shaft or rod 14 extending ar right angles from said arm 13 and that passes through said chain wheel and is connected to the slidable part 2 of crank lever.

The said rod 14 also passes through a slot 12a in the flywheel 12 that is located on the inside of the chain wheel, said rod 14 rotating with the flywheel so as to prevent distortion ofthis rod.

During the working or power stroke of the crank lever the cam follower 7, 7a travelling in the cam groove 8, causes the length of the crank lever to be extended, the axis of the pedal spindle 4 during this extension passing through a path indicated in dashcut line at 4b, the lever commencing to extend immediately the pedal spindle 4 advances from the top dead centre and returning to a non-extended position when it reaches bottom dead centre the maximum extension of the crank lever at which the torque is greatest and most needed occurring during downward movement of the pedal, e.g. approximately between the positions 4c-4c. This extension of the crank lever and the increased torque produced is predetermined so as reasonably to be within the capabilities of a user of the cycle. It is to be noted that the extension of the crank lever only takes place during forward and downward working movement (clockwise as viewed in Figure 1 ) and that the lever is not extended at top and bottom dead centre nor during its non-working stroke as the pedal spindle moves rearwardly and upwardly from bottom to top dead centre. The length of the crank lever at and between these latter positions remains constant during the non-working movement of the crank.

Although the invention is concerned more essentially with manually actuated apparatus or machines it may, with advantage, be applied to crank levers of power operated equipment.

Claims (8)

1. A crank assembly for bicycles or other machines or apparatus powered manually by pedals wherein each crank lever is adapted to be increased in length during its working or power stroke and is returned to a normal unextended constant length during its non-working stroke.

2. A crank assembly as claimed in Claim 1, wherein extension of the length of each crank lever during a power exerting movement is achieved between upper and lower dead centres of the foot pedal and the lever is fully extended while in its forward driving position and shortens to a normal unextended constant length before or on reaching bottom dead centre.

3. A crank assembly as claimed in Claim 1 or 2, wherein said crank lever comprises two parts one of which is movable lengthwise relatively to the other part so as to increase and subsequently decrease the effective length of the pedal lever, the movable and extendible part of the crank lever being operated and controlled by a cam or eccentric mechanism and being that to which foot pressure is applied.

4. A crank assembly as claimed in Claim 3, wherein said two parts of the crank lever are telescopic.

5. A crank assembly as claimed in Claim 4, comprising an inner part adapted to be fixed to the bottom bracket of a bicycle, and an outer part slidable over and capable of being extended from and retracted on to the inner part, the outer part having extending therefrom a member fitted with a cam follower constrained to co-act with a cam to effect extension and retraction of the slidable part ofthe crank lever.

6. A crank assembly as claimed in any of the preceding claims, including the provision of a balance or flywheel.

7. A crank assembly for bicycles or other machines powered manually by pedals, constructed and adapted to function substantially as hereinbefore described with reference to and as illustrated by the accompanying diagrammatic drawings.

8. A manually driven bicycle or other machine or apparatus fitted with a variable length crank assembly as claimed in any of the preceding claims.