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Gravity conveyor has many advantages. It is easy to be installed, expended and alternated in setup besides its low-cost. Theoretically when in use, gravity conveyor needs two angles of inclination. One is used to motivate the loading and the other is used to maintain the rolling of the loading(p3-2). As a matter of fact, it is impossible to have two angles of inclination at the same time. Therefore, we make a compromise angle between the more precipitous motivating angle and that of the less precipitous rolling one. As a result, problems will come out especially in two places(1) The angle of inclination designed for light loading will be put with heavy loadings. (2) After the first loading is motivated , the friction of the wheel will decrease because of the revolution which makes the second loading runs into the first one because its rolling speed becomes faster. After the bumping, the two loadings combines together making the rolling speed much faster. These two situations will result in the damage of the loading or hurting the workers as the loadings roll to the end of the conveyor and bump the quit machine. To solve these problems, we can use speed controllers , though it has certain restriction in weight extent and increases cost. Except for the problems mentioned above, gravity conveyor is the most economic ways to be considered in priority.

(1) use scope:

In terms of the articles with soft surface for example cotton bags, wheel conveyor can transport much better than roll one. In this kind of application , we can choose to put the articles in a line so that he articles can roll according to the traces. If in the bottom there are hollow containers or baskets, I is not suitable to use wheel conveyors. In order to keep balance in the process of transportation, one article should be supported at least by five wheels at any time(p3-4).

(2) the choice of material quality:

The framework of the wheel conveyors can be chosen from steel and aluminum. The aluminum framework can be used to load light and movable devices. Compared to the steel framework, its loading capacity is a bit low. And wheel can be made from steel, aluminum and plastics. Generally speaking, the loading capacity of steel conveyors is 11-23kg, that of the aluminum 4.5-18kg and that of the plastics is below 10kg.

(3) size specfication:

The valid width of the conveyors provided by most manufacturers is 300mm, 500mm or 600mm. And the standard length is 1.5m, 2m and 3m. The wheel number in unit length is decided by experience. Generally speaking, in terms of small goods, more wheels are needed, but the arrangement of the wheel consists of many different kinds of pattern due to the number of the wheels in each row and the interlace arrangement(3-5). The framework of the common standard wheel conveyor consists of two steels of 60mm?5mm.The wheel needs some oil to lubricate. If the using temperature is beyond 0-38℃, you should inquire the manufactures in case of certain lubrication problems. We also have specific wheel conveyors that cam be applied in higher or lower environment. The intensity and the loading capacity of the framework are related to the distance of the supporting frame. Most manufactures will provide a list of the permitting deformation extent(3-1). In the process of calculating, if the deformation extends the permitting deformation extent, you can consider add more supporting frames or use different ways of supporting. How much the conveyor declines is related to the weight of the goods and its surface conditions. As for cotton bags, the average inclination is 7.5cm high per 3m. If the goods are comparatively hard, smaller inclination is needed and the goods with comparatively soft surface acquire larger inclination. Experience and testing are the only way to decide the inclination. Generally speaking, the biggest length is 12-15m if there is no reducer.

支撑距离	每50 kg负载　变形量
支撑距离	钢	铝
1,500 mm	0.326 mm	0.602 mm
3,000 mm	2.414 mm	5.284 mm

(4) function module:

According to the function, wheel conveyor has many applying modules such as straight module, turning module and divergence module. The combination of different function modules can completely meet the needs of the system. As a rule, the turning module has patterns of 45°and 90? The average inclination of the wheel is only half that of the straight module. As the wheel of the turning module revolves independently and the wheel of the external part revolves faster, the radius of the inner part should at least the same as that of the length of the goods in order to keep the direction of the goods. Generally speaking, the longer the radius is, the more convenient it is to transport the goods but not to be filled in the turning section. The formula to calculate the width of the turning module conveyor is as follows(3-6).