Sariel.pl

Inspired by PistenBully snowgroomers. Features full independent suspension, dual longitudinal subtractor drive, pneumatic system with motorized valves and automated internal electric compressor, 3-section crane with infinite freedom of rotation, and lights.

Datasheet:

Completion date: 01/02/2009
Power: electric (Power Functions) / pneumatic (fed from internal electric compressor)
Dimensions:  length 84 studs / width 30 studs / height 39 studs (with crane in transport position)
Weight: 3.9 kg
Suspension: full independent with shock absorbers
Motors: 9 x PF Medium, 2 x PF XL
Pneumatics: two circuits with motorized valves, fed from internal electric compressor with a pressure switch and an airank

Somewhat spontanical construction, inspired by some PistenBully snowgroomers I’ve seen over winter. I wanted to model the entire functionality of such a snowgroomer, and this eventually turned out to require 11 motors. Here’s a full list of motors and the functions they control:

1. PF XL – drive (left tread)
2. PF XL – drive (right tread)
3. PF Medium – steering
4. PF Medium – pneumatic valve #1
5. PF Medium – pneumatic valve #2
6. PF Medium – crane rotation
7. PF Medium – lower crane section
8. PF Medium – upper crane section
9. PF Medium – compressor
10. PF Medium – flashing lights
11. PF Medium – front blade tilt

As can be easily seen, we have 3 motors more than the number of channels available with the PF system. To deal with it, we need to control these 3 motors by some other means. Namely:

1. both drive motors are controlled by a single channel
2. the flashing lights motor is permanently on, so it needs no channel
3. the compressor motor is controlled automatically by a pressure switch

The vehicle is suspended on 8 road wheels, each with an indepent suspension module with a shock absorber. There are shock absorbers of two different hardnesses used (first and last modules use harder shock absorbers than the middle ones). The front retracting wheels, and rear drive wheels are not suspended. The drivetrain consists of a dual longitudinal subtractor, where two PF XL motors drive the vehicle (one motor per one track), and the PF Medium motor controls steering by slowing down one PF XL and speeding up the other. It is, of course, possible to turn in place too.

A very unusual feature of the model is its pneumatic system. It consists of just two circuits, one with 3, the other one with 4 large pneumatic cylinders. Both circuits use regular pneumatic valves, which are motorized by PF Medium motors via safety clutches. The entire system is powered by a large electric compressor, that uses 4 small pumps divided into two units; when one unit is retracted, the other one is extended. It enables the compressor to operate more smoothly, reduces the vibrations it transfers to the hull, and makes the flow of air more fluent. The compressor is connected to an airtank, due to the high pressure needed straight off to operate the circuits quickly. The motor driving the compressor is controlled by a pressure switch, and thus the entire compressor module works in an entirely automated mode.  The construction of a pressure switch is strictly based on a concept created much earlier for the old pole reversers. Here I have just used the new PF switch instead. It is switched by two small pneumatic cylinders wrapped by a rubber band. Only the lower inlet of both cylinders is connected to the pneumatic system, therefore given a high enough pressure they overcome the rubber band and extend, turing the switch off. When the pressure drops, the rubber band forces the cylinders back, thus turning the switch on. I have used two cylinders due to a higher resistance of the PF switch compared to the one of the old pole reverser. The functioning of the pressure switch is shown in detail in both videos.

The front blade has one additional feature – it can be tilted left or right. The tilt is controlled by a PF Medium motor, via a safety clutch and a shaft that uses a number of universal joints to reach the blade’s head, where through a worm screw it rotates a vertical axle. Both front blade and rear smoothing plate are mounted on parallel levers, so they maintain their angle regardless of the elevation. I’ve tried to make the rear plate work more realistically by suspending it on two shock absorbers that press it against the ground. Thus the plate remains raised towards its front regardless of the ground’s shape. It does not, however, have the option to tilt left/right which is present in some snowgroomers.

Another important feature is the crane. It has the typical construction of small cranes (called HDS cranes in Polish technical terminology) used e.g. on cargo trucks and trailers: namely three sections, first vertical, two movable, with a hook at the tip. Two linear actuators are used here, both driven by PF Medium motors located inside the first, vertical section, without safety clutches. It provides precision and strength that pneumatics could not ensure. The crane’s first section is particularly complex and compact, due to space limitations imposed by the closely located battery box. One minor drawback of the crane’s design is the universal joint used to transfer drive from first to second section. It can’t operate when bent too much, so the third crane section becomes immobile whenever the second section is low. It is very inconvenient while connecting / disconnecting the load from the hook, but it provides load capacity far better than in a version with bevel gears instead of the universal joint (approx. 180 g compared to 50 g). On the video, the crane is carrying a 50-grams heavy truss module without even slightly bending under load. With loads heavier that 180 grams problems with the turntable’s stability begin to occur.

The infinite freedom of rotation is provided by a PF Medium motor located in the hull, and by the fact that there are only two wires going through the turntable. After a large number of revolutions these wires would eventually become entangled, but I assume that such situation is unlikely to happen during a regular utilization.

The last feature are the model’s lights, some of which are flashing. It is possible thanks to the solution used already earlier in my Tow Truck – a motor that drives the PF switch by an eccentric mechanism. The second video clearly shows details of this mechanism.

The model was well received, even though personally I did not consider it to be a project of a crucial importance. Perhaps the reason is that it was not my intention to try to use as much motors as possible, I simply wanted to model the functions of a particular vehicle. I liked, however, how well pneumatic system and linear actuators are compared in this single construction. The pros and cons of both these systems are clearly seen here – a pneumatically controlled crane would lack strength and precision, whereas LAs-controlled blade and plate would not adapt their elevation to the ground, not to mention how troublesome would it be to transfer the drive to LAs that would fold their wings. Thus, I have come to consider this model a good example of PF system’s potential not restricted by the 8 channels limit, as well as an excellent study of how pneumatics and linear actuators complete each other.

Photos:

Videos:

Media reference:

Klocki, Particolarmente urgentissimo!, TechnicBRICKs, YFOLe (Polish only)