Rotary Mowers - The Rotary Club

Nigel Churchin Machinery & Mechanics

Rotary mowers have been with us from the very start of grass mowing, the cutting principle behind the design is basically that of the scythe. A sharp blade cutting the grass and moving away to clear the area for the next cut.

Its simple layout allows most manufacturers to produce a rotary deck. By contrast, the more complex design of the cutting cylinder and the cutting unit restricts production to just a few specialist manufacturers only. So, with the wide selection of rotary models on the market, it's important to understand the options available.

Side, Rear or Mulch

There are three basic options on the type of deck or grass discharge you can choose from, and you can collect from two of them.

Side discharge decks are very popular in the US, the rest of the world and parts of Europe. However, they are much less popular here in the UK to the restriction of close cutting on the discharge side of the deck, and the hazard of flying debris from the discharge chute.

Side discharge is certainly a very efficient deck design; the grass is cut at the front of the deck, thrown across (always to the right-hand side), and exits the deck via the right-hand corner. It is the minimal distance that the grass is moved, which gives it the efficiency, but this also allows stones and other debris to exit the deck at high speed, hence the potential hazard.

Rear discharge decks allow close cutting to either side, with all cut grass moved from the front of the deck through to the rear discharge chute; often with the assistance of some baffles under the deck. This increased movement does reduce the efficiency of the air and grass flow under the deck, but it greatly reduces the chances of thrown debris exiting the deck. For commercial mowing in the UK, rear discharge is the standard option for these reasons.

A rear discharge deck reduces the chance of thrown debris

Mulching decks have been very popular, particularly on smaller machines, as they can leave a clean finish and remove the need for collecting grass clippings. They do, however, work at their best in lighter conditions and, for larger width decks, mulching larger volumes of lush wet grass can cause problems. There is also an additional power requirement to mulch for grass as opposed to cutting and throwing it.

The Blades

A rotary blade is supposed to cut the grass. To do this efficiently, it must be sharp! Rotary mowers often suffer from neglect. The blades are under the deck and, therefore, unseen. Blade inspection should be part of the daily pre-use inspection for every rotary mower.

The cutting blades should be visually inspected daily. They should be changed every week, with the worn blades being re-sharpened and balanced. Cycling blades in this manner will reduce wear and tear on the deck, reduce fuel consumption and increase productivity. The blades last longer as a result and all this saves you money. Changing rotary blades is easy and operators should be trained to do this.

Pre-use inspections: The daily inspection of the underside of the deck should note any wear or damage to the blades and other components of the deck, (baffles and anti-scalp rollers), and these should be reported to the workshop under a defects system.

Badly worn blades are also a safety hazard. Excessive wear can lead to blade failures and high-speed metal parts leaving the deck. The potential consequences of this do not bear thinking about.

A sharp, balanced blade / A worn, unbalanced blade will negatively affect cutting performance, and is a huge safety risk / The wear-cycle of rotary blades. If not maintained correctly, they will eventually break in half, with potentially devastating consequences

Lift: Most blades will have a lift tab at the back of the cutting edge, which is designed to create air flow under the deck. The airflow is designed to lift the grass up, so the blade can cut it more easily. It is also designed to carry cut grass away from the cutting area and out through the discharge chute. As the lift tab creates air movement it also creates noise, and manufacturers have a balance to achieve between the discharge performance and the noise the deck generates. Do check your operators and parts books to see if there are blade options for your rotary mower. There are often higher lift blades available which will improve the performance of the deck in heavy conditions.

Out-front mower decks: Many of the current out-front mowers sold in the UK have a service feature, which allows the deck to be swung through ninety degrees (to a near vertical position) for easy servicing and cleaning.

As many of the mowers are designed and built outside the UK, they are primarily designed for a side discharge deck. The rear discharge deck is, therefore, often a northern European accessory.

The key dimensional difference between a side and rear discharge deck is the depth of it. Rear discharge decks tend to be deeper (due to the discharge chute) than their side discharge counterparts.

This really matters when the decks are flipped up for servicing and for blade maintenance. Some models do not have enough ground clearance to allow the deeper rear discharge decks to swing into the service position, requiring the use of wooden blocks or ramps to access the underside of the deck. If it's too difficult to do, it simply doesn't get done.

An out-front rotary, which has been designed to have a rear discharge deck as standard, should be able to easily flip the deck for safe and easy service access.

Drive systems

Typically, rotary mower decks can be either mechanically driven, (via drive shafts, gearboxes and belts), hydraulically driven (by a single motor and final belt drive, or by individual blade motors). Each system has its merits.

Mechanical drives are usually more efficient, as less power is lost through the drive system. However, they generate more noise and vibration for the operator. They also require significantly more maintenance, lubrication and adjustment as belts stretch and wear. The greasing of PTO shafts and bearings can be time consuming, and as previously mentioned, if it's too difficult it simply doesn't get done as it should.

Hydraulic drive systems, particularly those with individual motors, represent the lowest maintenance currently possible. There is nothing to adjust or maintain, apart from the floating arms and perhaps the castor wheels the deck runs on. The disadvantage is some loss of raw power, as the drive is converted from hydraulic back to mechanical in the drive motors.

Hydraulic systems also benefit from relief valves, which prevent the system from being overloaded by the operator.

The Ransomes MP Series, built in the UK, features easy-service rotary decks

Height of cut adjustment

There is a wide range of adjustment mechanisms for rotary decks, ranging from hand wheel adjustment of the deck suspension system (very popular on mid-mount lawn tractors) which are easy to use, but perhaps not so accurate, to the individual spacers or multi-pin hole systems used by floating head units for close mowing on undulating ground.

The angle of the blade to the ground is critical to the performance of the deck. It should be either parallel to the ground, or ideally slightly lower at the front of the deck.

This means that the blade cuts the grass at the front of the deck and passes just over it at the rear. This reduces drag on the blade and allows for a cleaner discharge from the deck.

If the front adjusters are set higher than the rear, then the blade will effectively have to cut the same piece of grass twice, once at the front, then again at the rear, increasing the parasitic load.

Any grass cut at the rear of the deck is then likely to be pushed right around the underside of the deck before it can exit at the rear, further increasing the drag or loading of the deck. So, do always check the deck adjustments to ensure that it is either level, or slightly higher at the rear. The operator's instruction book will detail how the deck should the set, and the instructions should always be followed.

Washing and cleaning

The underside of a rotary deck can be caked in cut grass, mud and debris. Such build-up can adversely impact the discharge of the cut grass and, therefore, the output of the mower. Rotary mowers do need to have the underside of the deck pressure washed every week, to prevent build-up, and to allow blade replacements and a good visual inspection of the anti-scalp rollers and other components. Again, having a deck which easily flips up for service is key to making this process easy and achievable.

Given the environment which most rotary mowers work in, cleaning the underside of the deck is also a hygiene and personal safety issue, making regular cleaning and maintenance even more important.