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LAWN RAKES FROM A DESIGNERS PERSPECTIVE.

LAWN RAKE DESiGN / Parts of a lawn rake

    The parts for all lawn rakes change as one design becomes preferred above another. Perhaps the best place to start an examination is with the lawn rake handle which until recently has been fabricated in hard wood. Firstly, regarding the size, for many reasons the minimum length and diameter of an industry standard wooden handle is 48" and 7/8" diameter. Because the handle is an extension of the total rake length, during use the major stress point is normally about 3" to12" behind the handle socket where breakage occurs usually by splitting of the wood fiber grains which always runs parallel to the line of stress. Not all hardwoods have the same properties, not even in the same genera and these properties change drastically according to climate and weathering as the handle ages. Because of this wide variation in properties some rakes can be found with 1" diameter handles which under normal conditions the added weight and cost are not required.

     Recently retailers have lost their preference for the traditional tapered handle socket as illustrated on the left. The wonderful properties of tapers have been used for ages and following the apex of the industrial revolution, machinists had given them names such as Jarno and Morse. The functional theory of the lawn rake socket is the same as for machine tapers using the law of the wedge to compress the elastic properties of the materials to a point where friction can maintain the compression.

    Unknowingly some manufacturers have reduced the tolerances to a point where the taper can not maintain compression and friction of the joint which is lost according to the relationship the reduced surface area under the force through imperfections of the taper. Another reason the taper joint has lost favour is due to the expansion and contraction of the materials where moisture and temperature act mostly across the grain. On plastic rakes the problem is much more severe where the entire stress can locate on the tip of the taper and rapidly propagate a longitudinal crack in the plastic socket. With ergonomically designed handles of all types the problems can increase due to the additional torsional leverage or twist that can not be generated on a traditional straight handle.
    Untimely detachment of the handle can be the result of the socket widening. Some designers have allowed the socket to expand to compensate for substandard handle tapers while others weld the socket seam. Expansion of the socket changes the angle of the taper which is often compensated for by the surface ductility of the wood. Unfortunately if the rake is badly treated, the socket may be allowed to expand past the yield point and thereby permanently reduce the taper's ability.

    In an effort to find a solution, manufacturers examined the patent office records and found one. On the near right is the Bailie invention, patent 346,937 of 1934 and on the far right, a popular rake of the new millennium. Not only is the tine placement method essentially the same as the Bailie invention but also the tine stiffener spring is in the same form as it is today. This method reduces the chance of tine extraction and also clamps the handle to the socket mechanically using that old faithful Archimedean screw clamp. As recently as 1999 patents have been issued to the True Temper Co. which directly reference the Bailie invention as significant to tine positioning.
     Failure of the tines has always been a problem and they do a remarkable job considering their size. The type of steel can vary depending upon the manufacturer but most often a hard high carbon steel is used. It is lower cost and easier to manufacture than exotic alloys such

as titanium. The continuous tine rake shown on the far right is slightly heavier than the standard taper socket straight tine rake but has superior tine retention. A heavy lawn rake can be advantageous depending on the user and the type of raking being done. See ergonomics for more information.

     Methods of retaining the tines do vary. Automation of the assembly is reported to have been developed by the Douglas Company of Wyandotte, Michigan which was bought out by "Ames", another USA rake manufacturer. For many years a common method has been to notch the tines as illustrated on the left by the number 1. The tine notches are lined up in a row as in item 2, then placed in the clamp embodiment which also forms the handle socket. The embodiment is then welded along the seam shown in item 3, which is formed in an arch to cross the tines on an angle to prevent the centre tine dropping through the seam.

    The embodiment is bent to produce the rake angle, strengthen and add position control of the tines. The assembly is pressed to deform the steel into a depression shown above as item 4 thereby retaining the tines. Unfortunately, the clamp can be deformed by expanding rust or misuse sufficient to allow a tine to twist and move it's notch past the depression and become extracted. With the added space even the slightest torsional stress will remove the tines.
    The tine spreader bar controls the spacing between the tines and gives strength to the rake. Moving the tine spreader bar forward will change how each tine deflects under stress by moving some moments forward adding stiffness to the rake and thereby change the strike angle of the teeth. While most spreader bars are linear in construction some are formed in arcs. These alternative configurations and placements of the tine spreader bar control many of the properties of the rake by giving additional strength to the weaker or more active tine positions which suffer badly from twisting. Assembly of the tine spreader bar is normally through entrance slots for each tine and then folded over the tines. A tang at each end is then pressed over to clamp the assembly. The forward end of the tine spreader bar is formed in a radius to give needed additional strength and to graduate the transfer of stress between the components. However, the tine spreader bar can be easily forced out of position if not held in place by a tine stiffener spring or other means resulting in total failure of the rake. In plastic rakes the same features of the spreader bar are found moulded into the embodiment material using webs/ trusses/ couples and the like.
    To hold the spreader bar in position the tine stiffener spring is added. However the stiffener spring also changes the rate of deflection. Good rake action must allow the tooth of the tine to ride over the ground and not dig in when the rake is used with more force. The drag on the teeth forces the tines parallel with the ground plane. Without the stiffener spring the tines may go beyond parallel and allow the debris pile to push up and out from under the rake. Under light loads this is not a problem however. Placing the stiffener spring on the spreader bar could be described as adding a dynamic fulcrum giving a logarithmic spring rate to the tines. In plastic rakes much of this action is removed by preventing the teeth from penetrating the turf.

The tooth of the tine can greatly effect the abilities of the rake. Common variations include length and angle while plastic and alternative styles change the width, point, hook or position the tooth in parallel rows among other methods. Further, these styles can be formed in an arc as illustrated on the far left or in a straight row as shown on the near left. The green line indicates how the two styles contact the ground. The lawn rake is a complex assembly of elements and each method has advantages. Where the arc of the teeth is concerned, there is less torsional stress on the outer tooth and the contact angle is generally better, however the stress on the centre tines is far greater than the linear configuration. These features can be both good and bad. The linear configuration has greater area of the pan and requires less force for light debris. With good engineering and design both designs can produce a rake with almost identical properties through configuration of the total elements.

    Most ferrous metal rakes are coated with an epoxy based paint. The tines on some plastic/metal combination rakes have a chromate coating and the aluminum rakes are mostly anodized. Although lawn rakes come in many colours, for some reason green rakes are the most popular. However, the Ohio State University advises that blue, violet and green are harder to see and suggests that elderly users would benefit from bright painted tools. Designers must consider better leverage and improved grips to compensate for reduced agility. The introduction of new designs should not require increased cognitive abilities where a percentage of elderly homeowners have reduced short term memory making it more difficult to learn new skills. The university also points out that the elderly should consider carefully the use of power tools where their reaction times may be reduced and that the benefits of gardening result from a leisurely pace.
    That completes the introduction to the basic elements of the traditional lawn rake. For more information on alternative styles such as pickup rakes, adjustable width rakes, plastic or natural fiber rakes and more, please return to the index.