How successsful is the search for low-noise hydraulic systems?

Hydraulic systems have many applications and benefits, but few people would associate their use with a quiet working environment. By their nature, hydraulics make noise and the health and safety issues relating to noise have been recognised for many years and legislation is now placing clear demands on manufacturers to reduce noise levels.

The main source of noise in hydraulic systems is the pump that supplies the flow. Most of the pumps used are positive displacement pumps. Of the positive displacement pumps, axial piston swash plate type is mostly preferred due to their reliability and efficiency. A company that has taken positive steps to address this problem is Bosch Rexroth, which developed its Silence Plus generation of external gear pumps. The Silence Plus reduces the noise level by an average of 15 dB(A) compared with conventional external gear pumps. The pleasant, signifi cantly lower inherent sound is perceived in all pressure and speed ranges. Silence Plus results in a lower noise level for both the operator and other persons in the vicinity. This means that no costly, noise-reducing secondary measures are necessary, while the low-wear design also ensures efficiency and long service life. A non-involute rounded tooth profile, combined with helical cut teeth, forms the heart of the Silence Plus. Thanks to permanent tooth contact, the fluid is transported almost continuously and noiselessly. The possibility of noise from trapped oil between the tooth flanks is prevented in the first place. Just what level of performance the Silence Plus can achieve has been shown by its use by materials handling company Hiab Moffett in development of the the world's quietest truck-mounted forklift. In order to ensure the machine's operation could be as silent as possible a key area of focus was the gear pump. With Bosch Rexroth already supplying hydrostatic drives and gear pumps across the vast majority of Hiab Moffett's Truck Mounted Forklifts, Kevin Turnbull, Engineering Director, approached Craig Grant, Mobile Applications Manager at Bosch Rexroth, to see if the company could provide a solution. Speaking about Hiab Moffett's decision to develop a near silent Truck Mounted Forklift, Turnbull said: "There has been a trend in recent years, where deliveries are not just to typical building sites or industrial estates, but also to residential areas. With this increased move towards more residential areas, operators need to be aware of lower noise emission thresholds, particularly at night or in the early morning when most supermarket deliveries take place. Because of this we were seeing increasing demand for quieter fork lift trucks, and took it upon ourselves to develop a near silent version for those customers where keeping noise pollution to a minimum is a top priority." Speaking about the project, Grant said: "Having worked with Hiab Moffett for a number of years, we knew that noise pollution was becoming an increasing concern for the company's customer base. While there was a product on the market that professed to reduce noise emissions, it hadn't been specifically developed for mobile applications. Fortunately, when Hiab Moffett approached us we had just brought a product to market that, as well as meeting its demands for an electrically powered pump, operated with near-silent hydraulic functions." To demonstrate noise reduction potential of the new pump, Grant provided sound files of a traditional gear pump running at 1500 rpm across different pressures alongside the company's silence pump and the latest silence plus pump. Describing how the new Silence Plus Gear Pump is able to reduce operational noise, he says: "In a traditional external gear pump there is a single point non-continuous contact between the tooth and the groove, which results in pulsation of the fluid and it is this pulsation that causes much of the vibration that results in high operating noise. The reason we are able to reduce operation noise by an average of 15 dBA is that the Silence Plus Gear Pump features a round tooth profile, which allows fluid to flow continuously, as a result of permanent tooth contact. The reduction in pump vibration, which is typically as much as 75%, also reduces vibration in other components in the hydraulic system, which means it can make a significant impact on the noise of the machine as a whole." Developed specifically for mobile applications, and forklift trucks in particular, the Rexroth Silence Plus Gear Pump was highly commended in the Noise Abatement Society awards 2011, and provides a more robust bespoke design for use on moving vehicles. In order to ensure the new gear pump was up to the challenge of a mobile application, Bosch Rexroth added its own patented wear free axial force compensation technology to improve longevity. Since the initial stages of the project, Bosch Rexroth engineers have worked in collaboration with Hiab Moffett to create a working prototype which has since been awarded its own highly commended award from the Noise Abatement Society in 2012. Following on from the award success, the world's first virtually silent Truck Mounted Electric Forklift is now in pre-production. A different approach to the problem comes from hydraulics specialist R+L Hydraulics, which offers a system for reducing hydraulic equipment noise emissions. Consisting of a textile shrouding for hydraulic unit bell housings, the patented NRS system can reduce the total acoustic power output by up to 50 percent i.e. the equivalent of about 10 dB. The NRS system can be fitted without dismantling the hydraulic unit and is a cost-effective solution as original equipment and for retrofitting to existing hydraulic equipment. The pulsation of hydraulic pumps generates very strong vibrations that are not damped sufficiently by the aluminium bell housings commonly used today.Moreover, rigid bell housings do not provide acoustic insulation against structure-borne sound between the electric motor and the hydraulic pump. In unfavourable circumstances the thin-walled aluminium bell housing even acts as a resonance chamber that further increases the oscillation amplitude and thereby amplifies the pump's running noise. R+L Hydraulics was involved in a joint project aimed at developing a solution to this problem. The brief was to develop a practical, cost-effective system that could also be retrofitted to existing equipment with poor sound and vibration insulation. In tests, the NRS system – which has now been patented – reduced noise levels by 50 percent or the equivalent of up to 10 dB. The housing consists of a novel recyclable 3D "high distance" polyester fabric that is tear resistant, resistant against all commercially available hydraulic fluids and temperature resistant from -40°C to +120°C. The NRS system is available in four different widths and three lengths both as original equipment and as a retrofitting kit. In addition to the NRS system, R+L Hydraulics produces and distributes a comprehensive range of hydraulics accessories, such as heat exchangers, oil-air coolers, offline cooling units, rigid and damped bellhousings, aluminium and steel tanks, power transmission couplings and sintered metal products.