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HARDWARE SET

 

  • NOTE1: this project requires the build of projects GBOX and CONTROL BAR
  • EXTERNAL COMPONENTS The hardware needed to complete this project including the two projects above, can be purchased as a PROJECT SET by clicking HERE.
  • NOTE2:  requires of batteries which are not included.

OPERATION & ASSEMBLY TIPS

QUICK FACTS

  • Can be mounted on the ceiling or simply used between two supports
  • Make it as long as you want by printing any number of beams (male and female joint on each beam)
  • With the correct number of batteries and thread, lifts weight of up to 500g
  • PROJECTS used: requires the build of projects GBOX and CONTROL BAR

HOW IT WORKS

The structure of KRANE is formed by short and long beams that are framed by two supports connecting at each end. The beams have holes where long M4 screws can be passed through for mounting onto the ceiling and have the machine lift objects from there. Each beam has a male and female cylindrical joint so that KRANE can be made as long as desired by the printing of several beams (how about spanning the whole room?)

When assembled, the beams form a path for the base to slide along and support it in the vertical direction using a dovetail arrangement. The base and all that there is attached to it (including the lifted object) are suspended by the beams which effectively take all the weight. This is the reason why the beams are designed with cross sections that offer great resistance towards bending (more on BEAM theory coming up in other projects at LAYKANICS)

The base carries the system that reels in the weight. This system is composed of a support (or bearing) wall , a drum (on to which a thread is attached and reeled), a coupling and the gearbox containing the motor. The drum uses a high torque input to overcome the weight of the lifted object (tension in the string) and rotates to reel in the thread. In order to produce a high torque, a gearbox (GBOX) is required to increase the torque output of the motor at the expense of rotational speed. If the drum was coupled directly to the motor, the weight of the objects to be lifted would need to be very small. The coupling has a conical feature to increase friction and effectively transmit torque from the gearbox output shaft to the drum without sliding (see LEARN section). Without it, the weight of the lifted object would also need to be very small.

Finally comes the “spider” grapple. This sub-assembly is in charge of getting hold of the object that will be lifted. It connects to the drum through the thread that carries the weight. It has an arrangement of four arms connected to “grippers” with rubber tips (O-Rings). The grapple is positioned on top of the object to be lifted and lowered down . During this movement, the arms open as they contact the object until they get to a “feasible” grip position, then movement is reversed. The grapple lifts and the arms lock onto the object as gravity pulls it down. If friction is enough, the arms will hold the object and lift it to where we want it!

KRANE uses project CONROL-BAR to allow it to switch back and forth from a reeling (lifting) to an un-reeling (lowering) movement. Each support has two connecting ports where screws K1-F12 can be used to connect the cables coming out of the control bar to the cables that connect to the motor terminals.

To assemble, start by attaching the coupling to the output shaft of GBOX . Then place the drum into the cone. A distance of about 1 mm should exist between the mating surfaces of drum and coupling. If larger, sand the coupling conical surface and file the internal portion of the drum. Place the hex-nuts on the back of the coupling and the screws through the holes of the drum. Screw until both parts mate. Make sure the coupling arrangement does not slip from the output shaft by manually rotating. If it does, tighten both screws some more. Once a full grip is obtained, unscrew one of the bolts and attach the thread to it. Make sure that the back of the coupling is free and does not contact the wall of the gearbox. Slide GBOX and drum into the base dovetail. Assemble the bearing wall on the base and, from the outside, slide the shaft through the wall onto the drum. Pass the thread through the wall slot.

For the assembly of the grapple, pass the arm through the strut slot as shown. Then bring the arm bore to snap to the axis at the bottom of the strut. The arm should be able to rotate freely with gravity. If this is not the case, file down the lateral faces of the arm that could rub the strut walls and the arm bore. Pay special attention when removing the arm from the strut as this could easily break some of the arm supports. Attach the thread through the port on the center of the spider and by using screw K1-F12.

LEARN ABOUT FRICTION

You typically hear about friction being the source of all kinds of problems in engineering. While a lot of the time this is true, there are many instances in which we take advantage of it. In KRANE this happens twice and we would like to show you what kinds of tricks we use for this to be the case.

Friction is a force that only occurs between contacting surfaces. It happens along the contacting plane and always in the opposite direction to where movement will happen (in a static case) or is already happening (dynamic case). Friction depends on how strong the contact is between the two objects and it only appears to oppose movement. Before the contacting objects begin sliding, static friction reaches a maximum value then transitions to a lower once sliding happens. The formula to compute the maximum value of static friction or that of dynamic is:

 f = \mu_{s,d}N
f = friction force [N] , μs = static coefficient of friction [dimensionless], μd = dynamic coefficient of friction [dimensionless], N = normal force [N]

The coefficient of friction μ is determined experimentally by measuring the friction between objects of different materials or different types of surface finish. Its value is larger for static cases than for dynamic. The table below shows the values of the coefficient of friction for some materials of interest:

SURFACE 1SURFACE 2μsμd
Stainless SteelStainless Steel0.780.42
Stainless SteelRubber0.64-
PVCStainless Steel0.530.38
ABSStainless Steel0.350.30
PLAStainless Steel0.640.55
RubberRubber1.16-

The normal force N depends on the external forces bringing into contact the two objects. Since the value of μ is fixed, the only way to increase friction acting on an object is to increase the normal force exerted by the contacting body.

But why do we want to increase friction? In project KRANE there are two reasons:

1) SPIDER GRAPPLE: just like our fingers are able to lift things, the grapple relies on friction exerted on to the object to be large enough so that the object does not slip down. By using rubber tips (O-Rings) we have done all that we can to increase the coefficient of friction. The next step is to then try to increase the normal force. We do so by playing with trigonometry. When the spider is lowered on to the object, the arms rotate and increase the distance between their opposing tips just enough to accommodate the size of the object. When the spider is lifted back, the arms attempt to rotate in the opposite direction closing the distance between tips. Since the object is now on the way, the tips begin compressing against the object’s surfaces and, as a result, the normal force is increased. The higher the spider is lifted the stronger the compression and the larger the normal force. The larger the normal, the larger the friction. When the normal force becomes high enough to make the friction value equal to the weight of the object, the object is lifted. Do you think the angle at which the arms lift plays a decisive role in whether an object is lifted or not? Do you reckon there’s a maximum angle beyond which no object can be grasped?

2) COUPLING: in many mechanical parts, especially those involving rotation and torque transmission, it becomes impractical to add bolts (and holes) to secure one part on to another. An alternative force joining both parts must be sought and this is typically friction. A classic example is what we call “interference fit” between components. If a gear is to be assembled on to a shaft, the bore in the gear is intentionally made slightly smaller than the diameter of the shaft. When both are assembled, the diameter of the gear is elastically deformed to become the same size of the diameter of the shaft. The shaft exerts pressure on the gear bore and this exerts compression on to the shaft. The contacting normal between components is so high and the friction between the two components so large that they remain joined without one slipping on to the other. We can control how large we want the normal to be by making the gear bore smaller. This is extremely useful for transmitting torque and is basically the mechanism by which the gears on the gearbox are assembled to shafts K1-S2.

In the case of the drum in KRANE we go further and introduce an additional trick to get a larger normal. We use a “conical-fit”. The coupling has a feature with a conical external surface and an internal bore that assembles onto the shaft. The coupling features as the male cone. The drum, on the other hand, has a conical bore and becomes the female cone. Both cones are of the same height with the female cone slightly undersized. When put together, the cones touch at an offset distance, where both of their diameters are equal (SEE FIGURE) . Then the screws connecting drum and coupling are tightened and the offset reduced . As this happens, the female cone exerts a gradually increasing compression on to the male cone, which in turn begins compressing onto the shaft. (Notice that the slot splitting the coupling cone is there to allow for the coupling cone to be compressed). The normal and hence the friction between the coupling and the shaft is being effectively controlled by the tightening of the screws. With the conical fit, we converted normal pressure between components into a simple tightening action! To see how much undersizing of the drum cone we use with respect to the coupling, see the drawing section titled “FITS AND JOINTS” on sheet 4. Perhaps you can apply these same values to any of your designs where you want to transmit increased values of torque by achieving high friction between parts.

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