Safety Considerations and Design of Vehicle-Mounted Road Hedge Trimming Machine

　　In China, the construction standards for highways are high, with median strips commonly planted with various hedges, and slopes typically covered with grasses and shrubs for greening, guidance, and slope protection. These require frequent trimming.

　　Vehicle-mounted hedge trimmers are widely used for highway maintenance. They generally consist of a rotating base, boom, cutter head, power system, and control components. The cutter head is equipped with straight blades or circular saw blades, which rely on adjusted rotation to generate strong inertia for cutting.

　　The specific structure and technical level of these machines vary significantly, with considerable differences in configuration, performance, and reliability. Maintenance units should consider factors such as performance, reliability, and cost when selecting equipment based on their operational needs.

　　This article focuses on analyzing the advantages and disadvantages of different models from structural and configurational perspectives, providing a reference for maintenance professionals.

　　1. Boom Structure

　　There are two main types: telescopic booms and folding booms.

　　Folding Booms typically consist of three sections, as shown in Figure 1 below. They offer a long trimming reach, better suited for distant trimming of slope shrubs and weeds, often exceeding 5 meters. However, their drawback is insufficient rigidity—a common issue with folding booms. Due to the heavy cutter head, significant vertical vibration occurs during operation, resulting in uneven trimming. Operating speed is generally limited to 1–2 km/h.

　　Figure 1: Three-Section Folding Boom Hedge Trimmer

　　Another often overlooked disadvantage of the three-section folding boom is the difficulty of adjustment. Positioning the cutter head requires adjustments in four dimensions: three spatial coordinates plus angle (or levelness). The folding boom uses four hydraulic cylinders for adjustment (for the first, second, and third boom sections, plus the cutter head). Adjusting the first three cylinders affects all four dimensions of the cutter head, making it time-consuming to reach the desired position. Since highways often have bridges, culverts, and signposts requiring frequent obstacle avoidance, difficult cutter head adjustment significantly impacts overall operational efficiency.

　　Manufacturers’ demonstration videos often omit this aspect. Users who haven’t thoroughly researched beforehand may only discover this critical issue after purchasing the equipment.

　　Telescopic Booms were initially two-section designs, offering simple structure, good rigidity, and neat trimming. Since telescoping doesn’t affect the cutter head angle, they have one fewer degree of freedom than folding booms, making positioning easier and significantly improving operational efficiency.

　　To address the limited reach of two-section booms, manufacturers introduced three-section synchronous telescopic booms (see Figure 2 below) after 2017. These retain the advantages of telescopic booms while overcoming the short reach limitation and have been adopted by several manufacturers in recent years.

　　Figure 2: Three-Section Telescopic Boom Hedge Trimmer

　　2. Power System

　　There are two main types of power systems:

　　1. Direct drive by a four-cylinder diesel engine powering a hydraulic pump, with blades driven by a hydraulic motor. The advantage is lower cost, but the disadvantages include large size and weight, with the total machine weight often exceeding 2.5 tons, requiring a medium-duty truck for mounting and higher operating costs.

　　Furthermore, using a hydraulic motor as the main power source leads to rapid temperature rise in the hydraulic oil due to limited tank capacity and poor heat dissipation. When the temperature reaches 70°C, hydraulic motor efficiency drops significantly, necessitating a shutdown to cool down. This is unacceptable for highway maintenance clients.

　　2. Using a generator set, with cutting tools driven by an electric motor. This offers high speed and excellent cutting performance but at a higher cost.

　　The generator set typically requires at least 10 kW; higher power yields greater operational efficiency. The current top configuration uses a twin-cylinder, air-cooled gasoline generator with around 900cc displacement, providing an electrical output of up to 15 kW.

　　Twin-cylinder gasoline engines are air-cooled, compact, and lightweight. The total machine weight can be kept below 1.8 tons, allowing it to be mounted on a light-duty truck with double-row rail sides. This is an often-overlooked but crucial indicator for most maintenance units.

　　3. Cutter Head Structure

　　The mainstream cutter head structure features three straight blades arranged in a row, driven by a single electric or hydraulic motor. Blade lengths range from approximately 520mm to 650mm, providing a trimming width of about 1.5–1.8 meters.

　　A gap of about 5mm is usually maintained between the three blades, which can cause missed trimming. To address this, an inline tilted cutter head design emerged around 2016, where the cutter head is set at an angle of about 20 degrees to the direction of travel.

　　The shortcomings of straight blade heads are evident. First, they cannot handle hedge diameters exceeding 2 cm. Second, for continuous dense hedges, cut branches accumulate in front of the cutter head, leading to ineffective repeated cutting. In severe cases, the machine cannot advance, drastically reducing efficiency.

　　In 2018, a manufacturer introduced a new "dual rotating saws cutter " (see Figure 3 below), using two woodworking circular saw blades as cutters. The saw blades rotate at high speed, and the cutter head itself rotates at a certain speed, flinging cut branches away from the working face. This solves the repeated cutting problem, significantly improving operational efficiency—a notable technical advancement.

　　Moreover, by changing the action from "chopping" to "sawing," it produces clean cuts without splitting and can handle thick branches, achieving a performance leap. Relevant information can be found in pertinent patent literature.

　　Figure 3: Hedge Trimmer with Compound Rotating Circular Saw Blade Cutter Head

　　4. Control Method

　　The optimal solution combines wireless remote control + wired control. Button-type industrial remote controllers are commonly used, though some use joystick-type wireless remotes. Considering operational convenience and reliability, button-type controllers are more practical.

　　5. Safety

　　Hedge trimmers use high-speed rotating blades or saws for cutting. The poor operating environment and limited visibility for the operator can lead to accidental impacts ("blade strikes"), causing brittle fracture of the blades, which is not uncommon. Maintenance units must prioritize safety.

　　Blades are typically made from spring steel, requiring high standards of heat treatment. It is essential to use standard blades produced by professional manufacturers. Blades must undergo bending tests before leaving the factory to ensure they do not fail brittley.

　　6. Reliability

　　Highway work requires advance planning and official notification procedures. Safety isolation and configuring crash trucks are necessary before work begins, involving cumbersome procedures and high costs. Therefore, the failure rate of the hedge trimmer is a critical factor in selection. Choosing high-quality brands can minimize work interruptions due to equipment failure and save on repair costs. Maintenance units must remember not to opt for inferior products simply to save money.

　　Baoqing Li

　　Dep. of Technology

　　baoqing@equationtec.com

　　July 2, 2025