Laboratory studies of the performance characteristics of mowing mechanisms

Laboratory studies of the effectiveness of rotary cutting mechanisms when cutting single and groups of grass stems were undertaken. The effect of using static stem supports or ledger plates with clearances of up to 5 mm from the cutting blade was investigated. The effectiveness of cutting with single-toothed and plain discs was also examined. The effect of clamping the tops of grass stems was examined for both types of mechanism. A number of mechanism design parameters was investigated and cutting efficacy was assessed by measuring the stubble length of individual stems, together with the number of stems which were uncut, pulled out of the holder or broken at their base. The critical speed, which is the minimum required for efficient cutting, was also assessed. The use of ledger plates, above and below the cutting blade, resulted in lower critical cutting speeds than for conventional impact cutting when cutting single stems. For groups of stems, however, the critical speeds when using static elements did not differ significantly from those required for impact cutting. Clamping the tops of the stems resulted in very low critical cutting speeds (5 m/s or less). Critical cutting speed was reduced to below 35 m/s by increasing ledger plate length and by utilizing the whole of the blade length during cutting. Lower cutting speeds were also achieved by using a combination of ledger plate and blade angles which retained stems within the cutting area of the mechanism. If stems were at a distance of 10 mm or more from the ledger plate when contacted by the blade, a cutting speed of at least 35 m/s was required for effective cutting. In practice, the protrusion of the blade from the cutting mechanism should not be less than its height above ground level. Critical cutting speeds exceeded 80 m/s under laboratory conditions if stem inclination was large (greater than or equal to 65 degrees) or blade rake angle exceeded 60 degrees. This is equal to the cutting speeds required under practical conditions. A toothed disc cut groups of stems more effectively than single stems and at a low critical cutting speed. A plain disc cut effectively only if the tops of the stems were clamped. A minimum critical speed (20-27 m/s) was achieved with a toothed disc with teeth having a 0 degrees rake angle, a 30 degrees clearance angle and a pitch of 8 mm. Tangential wear of the tips of the teeth resulted in a proportional increase in critical speed.