While there is no doubt that there are excellent mowers on the market, a truly great mower cannot be judged on features alone – a great mower’s hallmark is the quality of cut it delivers, and that is a function of the blades alone. A fully synced, counter rotating reel, double sided bedknife and groomer are needed to produce a quality cut, throwing clippings evenly and preventing clumping. Note that the groomer lifts the grass before cutting, providing better accuracy and reducing damage to the grass.
The physics of shearing vs. impact
Rotary mowers cut by means of impact – the blade spins at a very high speed, contacting and impacting the stem, tearing it off. While that action may be perfectly acceptable for roadsides or rough grasses, it is detrimental to a golf green or sports turf.
Reel mowers work by a completely different mechanical design, and it is this difference that provides the shear cut of turfgrass stems. The reel, a rotating cylinder fitted with cutting blades, passes its blades against a stationary bedknife. The two blades act as a pair of scissors, cutting the turfgrass stem cleanly.
When a blade of grass is impact cut, the cellular structure at the tip is bruised and torn. The vascular system of the plant at the cut surface is also damaged, allowing moisture to exit the stem at a much higher rate than would otherwise be the case. This results in a rapid change in the plant’s moisture balance once the blade is removed. In addition, the wounded tip of the stem provides a much easier avenue for pathogens to enter the plant, greatly increasing the possibility for disease infection. Dollar spot, Pythium, and other turfgrass diseases will readily invade a bruised stem, taking over the plant’s vascular system well before the plant can mount a defense.
A reel mower’s shear cut leaves a much cleaner wound to the grass stem, with far less damage to the cellular structure. The natural defense mechanism of the grass is not hindered, leaving the grass blade green. Turfgrass researchers at university extension services have found that a grass blade cut by reel mower can heal itself up to 50% faster than a grass blade cut by an impact mower. The plant’s vascular system is not impeded, meaning a faster recovery from the stress of being cut. The increased surface area of the damaged, impacted blade also has a much greater opportunity to desiccate after being cut. Shear cut wounds close quicker, reducing the chance of disease invasion and water loss – two very important factors for the longevity and health of the turfgrass. This has direct implications on the chemical budget for the facility – the difference between needing to apply a chemical fungicide and not needing to apply a chemical fungicide.
Metallurgy and the math of ownership
Purchasing a commercial mower based on initial investment and not the longevity of the blades is a false economy. The metallurgy of the cutting unit dictates how often the blade can be reground, and the cost of regrinding is an important factor in the overall cost of ownership.
Superior reels are made from premium alloys such as Ni-Cr-Mo. This is not simply marketing speak; these metallurgic properties are important to the lifespan of the blade. For example, nickel increases toughness, chromium increases hardness and oxidation resistance, and molybdenum increases the strength of the metal at higher temperatures, as well as resistance to pitting and rolling from friction.
A blade with a higher Rockwell C hardness rating will maintain it’s edge for a longer period of time under the stresses of daily cutting. This reduced the time between lapping sessions, reducing labor costs per year for a fleet of six or eight mowers. For the serious turf care professional, the decreased interval between lapping sessions for a premium blade vs. a mid-grade blade can be the difference between a few thousand dollars worth of labor saved per year, or a few thousand dollars worth of labor lost.
The tempering process is just as important as the alloy mix, as it provides the blade with it’s required tensile strength and flexibility. Blades that were forged with the proper hardness rating along the length of the blade, but were tempered incorrectly will still show localized weakness along the length of the blade. A properly tempered blade will not show this weakness, distributing the tempering properties evenly along the length of the blade.
Equipment such as Baroness mowers employ this kind of metallurgy to produce cutting units that see extended use between lapping sessions, and earn a good reputation amongst superintendents and sports turf managers that care about their maintenance budgets.
The marcelling effect and frequency of clip
Most turf managers understand the relationship of cutting height to turfgrass quality. Fewer understand the relationship of frequency of clip (FOC) to turfgrass quality, and even fewer realize how it affects the quality of their cut at ground level when using a reel mower.
The frequency of clip is the distance between two consecutive blade contacts with the grass tip. Three factors influence the FOC – forward speed of the machine, number of blades on the reel, and the rotational speed of the reel. If the forward speed of the machine exceeds the capability of the reel to make consecutive blade contacts, a marcelling effect will happen. Any turf blade not contacted by the mower will remain taller than those that are contacted, creating the wavy appearance of a poorly cut lawn.
Marcelling is not always apparent on a golf green, but it is never desirable, and a poorly finished cut on a green is just that – a poorly finished cut. On a cricket pitch or lawn tennis court, marcelling will influence the response and roll of the ball, affecting bounce height and playability. The greater the marcelling amplitude, the greater the deviation to the ball’s behavior.
A 9-blade reel will provide a lesser FOC at a given RPM than an 11-blade reel. Increasing the number of blades on a given reel diameter allows the mower to make more blade contacts per minute, allowing faster forward speeds while preserving the FOC. However, there is a point where increasing blade count begins to negatively influence other factors, primarily engine torque and power requirements. As such, serious commercial reel mower manufacturers provide cutting systems as a package, taking engine speed, number of blades, drive ratio, and diameter into account when developing a new model. It is not an arbitrary choice to specify an 11-blade reel over a 9-blade reel.
Bedknife rigidity and closure management
The bedknife is only half the cutting surface, and the half that does not rotate is actually more important in determining the quality of cut. The bedknife must provide a rigid surface for the passing reel blades to contact, with a predetermined clearance (less than the thickness of a human hair on fine turf).
Any flex in the bedknife mounting system will increase the clearance at the point of flex, reducing the effectiveness of the cut at that point. On a single cutting unit, this results in striping inconsistencies as the blade passes over the turf. A properly mounted bedknife will allow for very tight clearances across the width of the bedknife, providing a more uniform cut.
It should come as no surprise that commercial-grade reel mowers have much heavier mounting systems for the bedknife than consumer-grade mowers – the increased rigidity is necessary to preserve clearances, ensuring uniformity of cut. While the lighter fabricated systems may contribute to a lower initial cost, the increased stiffness of the commercial-grade cast mounting brackets is required to maintain the micron-level clearance tolerances of the bedknife and reel blade.
Thermal expansion of metal is also a factor, as it influences bedknife clearance management throughout the day. A bedknife and reel set that are properly gapped in the early morning will be contacting by mid-day on a hot summer day. Proper thermal management dictates that regular clearance checks be performed throughout the day, particularly on hot days. Regular clearance checks can prevent undue scoring of the bedknife surface by the reel blades. They can also prolong the life of the bedknife and prevent accelerated dulling of the reel blades.
Adjustment ergonomics and real-world compliance
The best cutting geometry in the world is irrelevant if the required maintenance is not performed. This factor is frequently overlooked when specifying a reel mower.
Systems that provide tool-free or minimally tool adjustment to the bedknife mounting bracket allow for more frequent, precise, and easily performed adjustments throughout the mowing season. As a maintenance consideration it is entirely practical – two dozen extra turns of an adjustment rod over the course of a season is entirely within reason compared to the amount of labor saved by not having to wait until the bedknife has visibly deteriorated out of spec before making an adjustment. Conversely, two dozen extra turns of an adjustment rod over the course of a season may be an entirely reasonable way to ensure proper bedknife parallelism compared to the alternative of a loose, inconsistent, and generally dull bedknife that scores the bedknife surface and leaves micro-fissures in grass blades.
Power efficiency and hydraulic loading
There is a surprising efficiency gain to be found in premium cutting units that utilize premium alloy blades. In short, a properly maintained, sharp cutting unit requires less power to cut than a dull or improperly maintained cutting unit that has lost it’s ability to shear.
With proper blade sharpness and closure, the resistance of the cutting unit is decreased, allowing the powerplant and hydraulics to function with less strain than a cutting unit that is dull or out of spec. On a multi-head ride-on that utilizes several cutting units, the power savings are multiplied, reducing annual fuel costs.
The loss of efficiency on the dull cutting unit also has second order effects on the maintenance budget, as the hydraulics are put under greater stress to compensate for the inefficiency. This manifests itself in un-scheduled maintenance on the hydraulic circuit, usually at a much higher cost than it would take to simply keep the cutting unit in spec.
Floating suspensions and micro-contour tracking
Golf greens, and by extension sports fields, are rarely flat. A rigid cutting unit frame is unable to follow the contours of the surface, causing high spots to be scalped regardless of the skill of the operator. Advanced cutting unit suspension systems allow each cutting head to float independently, allowing the cutting head to track the micro-contours of the surface being cut.
The geometry of the suspension system and spring rates dictate how closely a cutting head can track the grade of the surface being cut, as well as how quickly the cutting head can respond to a change in contour. On a well designed cutting unit, this allows the operator to set the desired height of cut and the cutting head will track the contours of the surface to maintain that height. In contrast, with cutting units that have inferior suspension systems, the operator must constantly compensate for changes in contour to prevent scalping, which will adversely affect the FOC and reduce productivity.
