- Uses a 3-bladed Savonius wind turbine designed to rotate at a range of 500 to 2,300 RPM
- Employs a 3 shaft, perpendicular-axis transmission with a gear ratio of 14:1 and adjustable meshing
- Uses a fine-pitch steering system with a sweeping angle of 80 °
HOW IT WORKS
Depending on the velocity of air, the turbine on the T-ROVER can be set to move with a blow as little as that from your lungs (4 m/s) or through a constant stream of air like the one out of a hair dryer (12 m/s). It will also move if left outside on a windy day (7 m/s). This range of air velocities translates into turbine spinning speeds ranging from 500 to 2,3000 RPM’s (a car engine rotates at 3,000 RPM right before requiring a gear change). While spinning at fast velocities, the total power output of the turbine is only in the range of 20 to 500 mW, confining torque to a rather small average value of 32 g-mm. The transmission in T-ROVER has then the function to:
1) Increase the torque output to be able to move the vehicle through plane surfaces and slopes (of up to 20 ° inclination)
2) Transmit rotation from the vertical axis (turbine) to the horizontal axis (wheels)
The above is accomplished through the use of a worm gear which moves the spur gear in the upper shaft in a “screwing” fashion (shafts are perpendicular). At the opposite side of the shaft another gear meshes with the gear that is attached to the wheel axis, hence bringing motion to the wheels. The gear ratio (GR) of the worm-spur combination is 10:1 and that of the shaft gear to the axis gear is 14:1 (total torque increase from turbine to wheel axis = 10 x 1.4 = 14)
As learned in project GBOX an increase in torque will come along with a reduction in speed so unfortunately T-ROVER’s wheels spin 14 times slower than the turbine! It is key to keep attention to the spinning direction of the gears at the worm-spur arrangement. The turbine shaft spins clockwise as seen from above. The rotor-support (14-TROVER) at the bottom of the chassis restricts the axial movement of the shaft in the downward position but nothing prevents it from moving upwards. It must therefore be guaranteed that the reaction force that the pinion exerts on the worm is in the downwards direction (see picture) . This is accomplished by producing a thread in the worm that spirals in the clockwise direction.
As you will see in the LEARN section, a vertical wind turbine rotates regardless of whether the wind blows from the side, the rear or the front. This means T-ROVER will move towards you even if you set a blower in front of it!
On the front of the T-ROVER, left and right knuckles rotate each about an axis provided on their corresponding side rails (05 & 06-TROVER). The front wheels attach to these knuckles. A rigid bar links them together and forces them to rotate simultaneously in coordination about their own axis. This is the main principle behind the steering system of any vehicle. The left knuckle then connects to a grooved arm and rotates by following the transversal displacement of the arm in a cam-follower type of action. All that is left to do is to move the arm to be able to steer the vehicle and this is accomplished by another worm gear moving a rack (15-TROVER) on to which the arm connects.
Follow in order the steps below and combine with the information referenced in the Drawing to build T-ROVER:
Before starting, check that the following parts fit as intended with their mating counter-parts (use a metal file or cutter if you find the fits to be too tight):
– Strut (09-TROVER) into rails dovetail (05-TROVER & 06-TROVER)
– Rear wheel (03-TROVER) into Shaft (K1-S3)
– Middle strut (10-TROVER) into Rack T-Slot (15-TROVER)
-Shaft (K1-S2) in bearing holes at rails (05-TROVER & 06-TROVER)
-Shaft (K1-S2) in bearing holes at platform (01-TROVER) and at Rotor Support (14-TROVER)
-Torque Cob (11-TROVER) into Pinion Gear (K1-G10) on top of Turbine Rotor Shaft
– Turbine (02 – TROVER) into Torque Cob (11 – TROVER)
2.SHAFT AND GEAR CONSTRUCTION
Assemble the four shafts arrangements following the distances and orientation specified on page 2 of the Drawing. From right to left in the PICTURE you will end up with: a turbine rotor, a steering axis, first transmission axle (upper shaft), and the second transmission axle (made of two 3MM shafts: the wheel axis with gear – K1-G48- and a wheel axis without).
To assemble the turbine rotor, you will need to first get the worm gear (K1-GW7) inside the 2MM shaft at the specified location. You will find it very difficult unless you use a suitable the TOOL like the one we have designed at LAYKANICS for this purpose ( go to BACKYARD ->TOOLS and print it). This tool encapsulates the worm gear and lets you push it down into the shaft by pressing down on the wings at each side. Also NOTE that you must insert the Rotor through the T-ROVER platform (01-TROVER) before getting the last pinion in (K1-G10). This is done to lock the rotor vertically.
To assemble the steering wheel you need to print both halves of the worm gear (12-TROVER, 13-TROVER) and the corresponding Steering Handle (17-TROVER). Both halves fit into the Handle through an hexagon protrusion. Then a nut (K1-NT) goes into the cavity formed by the two halves (page 2, Drawing) and a screw (K1-F12) is used to lock all 3 parts together. Finally, a 3MM shaft must be slid through the central hole (K1-S3). NOTE: make sure the shaft sticks out by the distance specified in the drawing, otherwise it WILL NOT FIT the cart. If it does not, remove any excess of material and fit again.
3. THE TRANSMISSION
The two shafts that form the second transmission axle are joined by means of a coupling (08-TROVER) with nuts and bolts (see drawing, page 1, items 8). You must keep this into consideration when following the steps below:
Insert the upper shaft and wheel axis on to the left side rail 05-TROVER as shown . Bring the right side rail 06-TROVER into position and close the frame with strut 09-TROVER at the front and rear. Manually turn the upper shaft with your hand and notice if it rotates freely. Disassemble the frame to get the wheel axis out and with a file open up / slightly the bearing holes on 05-TROVER. Once smooth rotation is obtained, lock the rear axis in position by assembling the back wheels as shown. . Use bolts (K1-F18) and nut (K1-NT) x 2 to secure the coupling onto the wheel axles.
The next step is to set the turbine rotor in place. Assemble the platform on top of the chassis frame. Insert the platform tongue in the strut slot and use screws and nuts on both sides. DO NOT tighten the screws completely. From the bottom, insert the turbine rotor through the platform hole (long side of the turbine rotor). Slide the platform axially until the rotor meshes with the pinion on the upper shaft. Then assemble the rotor support 14-TROVER and slide axially until the turbine rotor is completely vertical . Test the rotation manually. It should be smooth and complete (no gear disengagement). When finally adjusted, tighten the screws on the platform.
4. FRONT WHEELS
Insert the right and left knuckles on to their corresponding side rail and screw the wheels on to them using the long fasteners K1-F12. These should be tight enough to prevent the wheel from wobbling but left a bit loose so as to get free rotation of the wheels. Snap fit TIE ROD (16-TROVER) to the Knuckles as SHOWN in the picture .
5. STEERING SYSTEM
Insert the steering axis into the cart by placing it an angle and getting the shaft tip into the hole of the rail on the left of the picture. Then snap fit the handle into its position on the right hand rail on the picture. Lock the handle by screwing in the Steering Lid (18-TROVER) into the rail.
Mesh the rack 15-TROVER with the worm gear in the steering axis and from outside, slide the middle strut (10-TROVER) through the T-slots on the side rail and on the rack. Then use two screws to fix the strut to the chassis tightening until meshing of the worm gear and the rack is not too tight. NOTE the left chassis has a built-in spring to accommodate for axial adjustments if meshing of the gears is not correct. Test by manually turning the handle and guaranteeing a smooth transversal sliding of the rack. NOTE before completing this step, file down the teeth on the rack (15-TROVER) after printing to make sure they mesh properly with the worm gear.
Finally a long screw (K1-F12) to attach the Tie Rod (16-TROVER) to the rack (15-TROVER) making sure it connects to the left knuckle. Snap fit the steering bar (16-TROVER) to both knuckles and by now you should have a full vehicle assembled!
Assembly of the turbine is the last step. First insert the Torque Cob (11-TROVER) into the top of the Turbine Rotor . The Torque Cob locks with the rotor through the teeth of the Pinion. Then press the turbine (02-TROVER) into the Torque Cob as shown. The turbine and the cob WILL have a WOBLY fit but this helps to fast free rotation.
The idea is that you can replace the turbine with one of your own design without having to disassemble the vehicle at all. As long as you keep the dimensions of the hub (page 4, Drawing), you can design any turbine of your choice and use it with T-ROVER.
Wind turbines are a growing solution for harnessing the free energy contained within the wind. From powering entire cities to producing electricity for small houses, wind turbines are of different sizes and types. The two major types of turbines are Horizontal Axis Wind Turbines (HAWT) and Vertical Axis Wind Turbines (VAWT). The first type is most commonly seen at wind farms and offers advantages over VAWT in terms of construction and operation. However, HAWTs need to be aligned in the direction the wind is blowing in, requiring complex systems to do so. Vertical Turbines however have the advantage of operating regardless of the direction the wind blows. In order to highlight this advantage, we use them with T-ROVER.
VAWT’s design fall under two categories: Savonius rotors and Daerrius turbines . Although both extract energy from the wind, each makes use of a different principle to rotate. The Savonius rotor offers a drag surface for the air to exert a force on to. The Daerrius type, on the other hand employs aerofoil lift as the force for rotation. We shall cover lift at other projects in LAYKANICS and will focus on the Savonius rotor here.
The power a turbine can extract from the wind depends on the size of the turbine (swept area of the turbine = diameter x height) and, most importantly, on the velocity of the wind. In any case, the maximum power that a turbine can produce follows the formula below:
P = Power [Pa], Cp = Power Coefficient, ρ = Density of Air [kg/m3], A = Swept Area by Turbine [m2], V = Velocity of Air [m/s]
The Power Coefficient (Cp) is a parameter introduced to denote how efficient can a turbine extract energy from the wind. Some turbine designs are able to extract less power than others. In the ideal case of a perfect turbine design, Cp is limited to a maximum value of 0.593 by physical principles (this is known as the Betz limit). Savonius rotors usually operate at power coefficients between 0.1 and 0.17. Another parameter that characterises a turbine design is the speed ratio defined as:
λ = tip speed ratio, ω = rotational speed [rad/s], r = Turbine Radius [m], V = Velocity of Air [m/s]
The “efficiency” or Cp of a turbine depends strictly on the relation of the tip speed at which a turbine operates to the wind velocity under which will be operated. This varies from turbine type to turbine type but, in general terms, a turbine is more efficient if designed to rotate faster. The graph provides Cp as a function of λ and is an excellent point of comparison between turbine types.
For T-ROVER, we chose the design rotational speed to be: 500 RPM, a diameter of 85 mm, and a blade height of 40 mm. Also, we have designed it to operate under an average wind velocity of 4m/s. With this information and the graph above, you can determine that λ = 0.56 and Cp = 0.15. The maximum ideal power to be obtained follows the formula:
So we aim to produce a maximum power of 19 mW. At a design spin speed of 500 RPM (52 rad/s) the Torque to extract out of the turbine is:
And with this torque value we can go ahead and design a vehicle around the turbine. Knowing that the transmission has a 14:1 gear ratio, the total weight of the rover is 70g and the wheel diameter is 36 mm, do you think you can determine:
– How fast the T-ROVER will run (mm/s)?
– What would the maximum slope angle it could overcome be?
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