Total weight of the conveyor comes to 591.2kg
Calculations
1. Determining Force on tail bearing
Tt = 2368.6 N
Summing forces in Y direction:
Tt*sin25 ° - 0.5mg = 912.7 N
Where m is mass of pulley
Forces in x direction Tt*in25 = 2146.7 N
Magnitude of force on the bearing = √ (912.7^2 + 2146.7 ^2) = 2332.7 N
Total force on each bearing on the tail pulley is 2332.7 N
The life of this bearing is given by the equation
L = (C/P) ^k * 10 ^6
Where k = 3 for ball bearings
L = (10.8 / 2.3327) ^3 * 10^6 = 99.24 * 10^6 revolutions
This is the similar for all the bearing in the design as they all have the same dynamic capacity.
The number of revolutions needed over the period of 5 years is given by
105 * 60 * 4000 = 25.2 *10^6
Where revs/min = 105
And 4000 is the total number of hours
Determining force on the head pulley
T1 = 2951.7 N
T2 = 2430.4 N
Te = 521.3 N
Force on the bearing due to weight of the pulley = 0.5 mg = 88.3 N
(Since weight is divided between both bearings)
The resultant force acting on the bearing:
|F| = √ (521.3 ^2 + 88.3 ^2) = 528.7 N
Excel workbook showing the important tensions in the belt at the specified belt speed and idler spacing.
The stress in the shafts reached a maximum of 93.7Mpa in the motorsleeth, and 87.8 Mpa in the driven axle. These are both safe even if a safety factor of 2 is taken
Details of the design
Idler spacing
We input all the constants into excel, then determined a velocity which would keep all the tensions under 3 KN, thus we used solver to work out the most efficient spacing of the idlers. This showed to be close to 1.665m, this meant we needed to use 5 idlers on our design. The first idler had to be reasonably close to the head pulley as the way we mounted the head pulley required this. There also needed to be a idler close to the tail pulley because if the idler was after the feed chute too much material would fall back. All the idlers were evenly spaced in between.
The return idlers were strategically placed as to limit the overall sag of the belt, this needed to happen because the more sag in the belt the larger the power requirement to return the belt.
The head pulley
When trying to figure out where to place the head pulley we contemplated a few different setups, mounting it using a pillow block on top of the frame, or below the frame using a pillow block. The other option was to cut a hole in the frame and mount the shaft using flange bearings. We decided to go with mounting it underneath the frame; this was because it requires less work to use a pillow block than to use a flange plate, as the frame has to be cut. Also the reason we mounted it on the bottom was because we realized if we mounted it on the top the end of the conveyer belt would be sloped up because the top of the head pulley will be higher than the rollers on the carrying idler. This is undesirable as the head pulley is the drop off point of the material and it is much better to have a downward slope to help the material come off the belt as opposed to an upward slope which makes it harder for the material to come off the belt.
The take-up
Mounting the take-up proved to be a tricky job due to the C-section facing outward. To overcome this problem we cut the top part of the C-section off, this only needed to be a 400mm section. Now the take-up can be mounted onto the frame.
The power transmission system
After determining the power requirement for our design we selected a motor from the Renolds catalogues. We worked out that the output of the motor needed to be 860 W at 105 revs per minute to achieve the required belt speed. So we selected a motor with a GMOD frame. This motor supplied 1.1 KW of power at 103 Rev per minute. In order to get the power to the head pulley we contemplated a number of arrangements of the motor. This includes a V-belt drive, a chain drive and a bevel gear setup. In the end we choose a chain drive as this allowed us to keep the motor a distance away from the head pulley and have the motor and driven shaft for the pulley in different positions. Also we didn’t need any precision mounting, as bevel gears were not used. After selecting the method of transmission we went on to choose the particulars of a chain drive. We consulted the Saeco catalogue, and looked at the methods of selection. Using the selection method we choose a small sprocket and worked out that it should be the O8B-1 sprocket that has a 12.7mm pitch, has 25 teeth and is 101.33 in pitch diameter. Then using the speed increase ratio we selected the large sprocket for 26 teeth, this meant we had to have a pitch diameter of 105.36mm. For these two sprockets with the pitch of 12.7mm we need an OCM08B chain that has the same pitch length. This was a single chain design. The number of link required to get a shaft distance as close to 600mm was 120 links. This gave a shaft distance of 600.7. The required setup needed to be lubricated every 2-3 hours so we put a drip feed. To attach the motor to the drive sprocket we needed to add a link so we designed a part which we called the motorsleeth. This allowed the motor to link to the sprocket through tight fitting shaft. The shaft has an interference fit with keys, so to be put together they have to be squeezed in tight. The fit employed was the H7-n6. The same thing was needed for the driven shaft to the larger sprocket and the same fit was used. To attach the driven shaft to the frame we used pillow blocks, these bearings allowed us to mount he shaft onto the frame and the bearing met the life requirement in the specification of the problem.
Mounting the motor
When mounting the motor we encountered more problems, this was where to place the motor. We contemplated different position when designing the transmission system and went with the motor being mounted below the frame facing the side, as to line up the two sprockets. But the problem occurred when we realized that the belt would get in the way if we mounted the motor onto the frame. Also the frame was too small to mount the motor onto. So we designed a motor mount that consisted of C-section mounted onto the C-sections of the frame. Between the two lower ends of the C-section we placed a plate called the mount plate. This was held together by bolts through two plates that joined the C-section and the mount plate. This plate was called the bolt plate.
Safety
When using a chain drive it is necessary to consider the safety of the wheels spinning. It is very easy for someone to accidentally place a body part into the chain and cause them injury. To overcome this problem we designed chain case to seal off all the unsafe parts of the power transmission. This is the sprockets and the chain.
We also designed a pulley cover so people could not get injured, by getting caught in the pulley. Also covering the take up meant only expert could modify the take up, as the pulley cover has to be removed. The pulley cover is easily removed as it is mounted my two bolts attached to the frame place.
