36 n MECHANICAL POWER TRANSMISSION June 2025 www.drivesncontrols.com The pitfalls of shaft-mounted gearboxes – and how to avoid them Traditional gearboxes with solid shafts have several drawbacks – including limited flexibility due to their rigid design, weight and susceptibility to vibration or wear – all of which can result in high maintenance costs and poor efficiency. Overcoming these shortcomings is where shaft-mounted gearboxes – also known as direct-drive gearboxes – come in. These slide over the shaft so the shaft supports the weight and torque. There are numerous advantages to this approach. First, shaft-mounted gearboxes eliminate the need for extra mounting components such as couplings or belts. Not only does this make the designs simpler, it also eases assembly and alignment and makes the systems ideal for space-constrained environments. Known for their strong performance in tight spaces, shaft-mounted gearboxes convert high-speed, low-torque motor output efficiently into low-speed, hightorque motion, reducing misalignment and the need for maintenance. That’s why these gearboxes are popular in critical applications such as conveyors, mixers and pumps. But shaft-mounted gearboxes come with their own set of installation challenges. If they are not mounted properly, they may operate inefficiently or fail prematurely. Here are some tips on how to avoid such mistakes and, in doing so, achieve the highest efficiencies and cost savings. n Ensure proper alignment Many operators make the mistake of bolting the gearbox to a solid plate or mounting surface. This prevents the gearbox from adjusting to shaft movements, resulting in misalignment, vibration, and damage to components such as bearings and gears. Over time, even slight shaft imperfections can lead to premature failures. Proper alignment is essential for the smooth operation and longevity of shaftmounted gearboxes. A reaction bracket, torque arm or reaction bolt kit can allow the gearbox to move with the shaft. These flexible mounting systems help to accommodate misalignments, reduce radial loads and minimise wear on the gearbox bearings. In large machines, such as mixers or conveyors, the reaction bracket helps to redistribute torque and protect the gearbox from damage. The anchor point for the reaction device must be strong enough to handle high reactive forces, especially during direct-on-line (DOL) starting when torque can spike up to three times its nominal rating. Operators should also schedule regular inspections during installation and maintenance. n Choose the right components Selecting the right components is critical when designing a gearbox system. It’s essential that the shaft and bearings are properly sized to support the gearbox’s weight and torque. The chosen parts must help the system withstand operational loads without premature wear or failure. For instance, we recently helped a customer in the asphalt industry who had experienced repeated gearbox failures on its mixers. The issue arose because two gearboxes were bolted down solidly on misaligned shafts. The bending of the shafts under the weight of the gearboxes caused excessive strain, leading to the frequent breakdowns and costly downtime. The mixer used a 37kW motor which produced significant power and was crucial in ensuring that the gearboxes could handle the torque demands. To solve the problem, we designed a new system using a reaction bracket and flexible mounting, along with Bonfiglioli hollow-bore gearboxes. These are popular in quarry and mine applications where reliability and low maintenance are desirable. This approach allowed the gearboxes to shift as the shaft moved, alleviating the stress that was resulting in the premature failures. The user experienced a significant reduction in failures. The previous system was failing two or three times a year. The new one has run smoothly for years without issues. n Consult technical experts This example shows that choosing the right gearbox components is only part of the equation. It’s just as important to consult an expert who understands how best to configure these parts for your application. This helps to avoid installation mistakes – such as bolting the gearbox solidly or using incorrectly sized shafts – which can lead to significant issues further down the line. While shaft-mounted gearboxes offer clear advantages over traditional gearboxes in some applications, their efficiency and longevity depends on proper installation. n Although shaft-mounted gearboxes offer numerous attractions, especially where space is limited, their performance can be compromised if they are not installed correctly. David Strain, technical director at Technidrive, explains some common mistakes when installing these gearboxes, and how to avoid them to unlock efficiency and cost savings. Shaft-mounted gearboxes are popular in quarry applications where reliability and low maintenance are desirable.
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