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Risultati finali
Internal/external gear pairs can be used to produce compact gear units and the tooth loading should favour plastics. A gear rig was designed and built to test this proposition. For economy and to reduce lead times, an existing CTPD product was used as the basis of the test gear, and bespoke metal pinions were made for the rig. The considerable body of data and some test rigs developed in previous non-EU funded projects was also made available to Progear. As much of this work was based around spur gears of 60 mm. centres the work was carried out on comparable gears. The benchmark was ASK2348/1S 30z 20pa, a 2module standard spur gear of 1.65 contact ratio (CR).
In order to examine the effect of tooth size upon performance a smaller tooth is specified in ASK2348/20 60z 20pa, a 1mod. Standard spur of half the tooth size, but of similar PCD but 1.78 CR.
It is well known that gear performance is related to smoothness of torque transference between mating gears, and that integer contact ratio (CR) gears promote this property, ASK2348/18. 30z 20pa, 2mod. spur having a long tooth has been designed. This serves to compare state of art in the USA as AGMA.XTP3 where whole integer contact ratio by tooth lengthening has been specified. Another way by which integer CR can be achieved is by varying tooth pressure angle as embodied in ASK2348/17 30z 13.7pa, a 2mod. Spur gear.
It has been shown that under load, when gear teeth deflect, the smoothness of operation is promoted by the introduction of tooth tip relief. ASK2348/24 30z 13.7pa, a 2mod.spur design adds this feature to the previous gear to check the effect of tip relief.
Another way of obtaining integer CR is by introducing a helix angle such as on ASK2348/19 30z 20pa 8deg., a 2mod. helical gear. It is known that introduction of a helix promotes air/lubricant flow through the teeth which reduces temperature and reduces noise impact. The previous designs for whole integer CR have aimed at a CR of 2. Where greater power is to be transmitted, a still greater integer CR of 3 has been designed in ASK2348/25 26z 20pa 30deg. a 2mod. Helical long tooth gear.
Finally there will be an effect upon performance by changing the transmission ratio and to sample the effect of this ASK2348/16 20z/40z 20pa, a 2mod.standard spur has been designed.
This suite of gears cannot completely simulate the whole range of design possibilities that exist, but is intended to facilitate research into the effect that some of the variations will have upon noise and power density involved.
When Rover withdrew from the project some refocusing of the efforts towards noise tasks was implemented.
Two lists of materials were proposed for tasks 5 (noise and vibration reduction) and task 6 (power transmission). A model compound was selected: polyamide 66 + 15wt% PTFE +30% Glass fibre. The compatibility between various liquids (potential external lubricants) and different polymers used in the field of gears was also presented.
The evolution of surface morphology and wear debris as a function of testing time helped to visualise the formation, through melting of the polymer at the surface and breaking, of a transfer film layer between the two running gears. An attempt to measure the surface temperature through the thermal behaviour of polyamide matrix indicated a value of about 215 degrees Celsius, however, this value is probably underestimated. When dissimilar materials are run against each other the melted polymer film is not transferred from the counter surface but comes from the tooth bulk material. Analysis of tooth breakage and observations on other polymer matrices such as various polyamide based composite gears, polyacetals, finalised the study on wear mechanisms.
Changing compound composition (internal lubricant type, fibre type, etc.) and changing the injection moulding process (dual phase moulding) were proposed to improve gear performances. Until now, these different approaches contributed to increased understanding of wear mechanisms but did not clearly bring significant wear reduction. Two rapid screening methods have been evaluated: the pin-on-disk tribometer and the two roll disk rig. The tribometer shows interesting results that are close to the direct observations on gears.
The results obtained contain information about how the noise production of gears can be influenced by the choice of gear material (combinations) and how it is related to speed and transmitted torque. In combination with the outcome of the wear tests this information can be used by designers to choose the optimal material for the specific requirements of a transmission.
The knowledge gained, experience of noise measurements and analysis methods are applicable to other acoustic problems on any product or subsystem in copiers, printers or actuators. This knowledge will be spread through courses and demonstrations.
The reduction of noise and vibration level of office equipment and actuator systems is driven on one hand by the need to strengthen the industries competitive position in international markets and on the other by the need to gain entry to new markets where high noise levels restrict the application. In maturing markets, the discrimination between products shifts from functionality and price to the meeting of environmental standards. Products developed for industrial environments will find new applications in office or medical environments when they can meet the stricter standards. Some noteworthy results are:
-Doubling the speed leads to an increase of overall noise pressure level with 6-7 dB for all materials, loads and gear geometry.
-Surface noise between the sliding gear flanks, due to slip stick effects, occurs between gears made of the same materials running against each other without internal or external lubrication. The frequency spectrum of this surface noise is independent of speed and increases with the load applied. The frequencies of the surface noise are high above the mesh-frequencies of the gears and their higher harmonics and can contribute 10dB to the total noise level.
-Soft materials when lubricated, or when run against steel or a harder material, perform well at low loads. At higher loads the tooth deflections lead to high transmission errors and noise production. Some POM-based compounds performed better at higher loads.
-All noise measurements were performed during a linear speed increase from 100 to 3000rpm. The noise level graphs show clearly distinguished peeks when meshing frequency corresponds to rig resonance frequencies.
-If the design allows it, a minor change of speed or in the number of teeth (e.g. by changing the module) gives, in the case of resonance, more improvement than choosing different materials or tooth geometry.
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