Article citation information:
Petroci, J., Mantic, M., Kulka, J., Boslai, R. Design of a compact dual-purpose starting clutch in the drive of a prototype vehicle. Scientific Journal of Silesian University of Technology. Series Transport. 2016, 91, 107-112. ISSN: 0209-3324. DOI: 10.20858/sjsutst.2016.91.11.
Ján PETRÓCI[1],
Martin MANTIČ[2],
Jozef KUĽKA[3],
Róbert BOSLAI[4]
design of A Compact Dual-Purpose Starting Clutch IN THE Drive of A Prototype Vehicle
Summary. Initially, the development of a dual-purpose
clutch was based on racing experiences and application requirements, as well as
the results from testing the new power unit in the existing prototype
vehicle. In order to achieve the highest possible driving range of the prototype
vehicle, it has been necessary to eliminate the maximum possible losses and
drive in unnecessary components. The design aimed to achieve simple access,
reliability and low weight.
Keywords: centrifugal clutch, prototype,
engine
1.
INTRODUCTION
The internal combustion four-stroke
engine Honda GX 25, which is designed for small agricultural and garden use, involves
a manual starting system. Output power is transmitted through a simple
centrifugal clutch mounted on the flywheel. A crank shaft on the starter
mechanism side is designed with a relatively low metric 6mm thread and a surface
with a diameter of 11mm, meaning that it is not sufficiently sized to
transmit the torque of the electric motor used for starting the prototype drive
unit. Applicable electric motors for radio-controlled cars generate high stress
on the crankshaft as well as on the gear mechanisms, which in turn breaks the threaded
part. Automotive-type starter motors, which are used in small motorcycles with
a sliding pinion gear, are also unsuitable given the small dimensions of the
thread shaft. In other words, when the slide pinion bumps into the starter
flywheel, a bending force impacts on the crankshaft, resulting in bending and
breakage. Basic theoretical principles, which are necessary when addressing these
matters, are described in [1], [2].
2.
CONSTRUCTION
The crankshaft is shaped on the
outlet cone and features a key for the precise assembly of the flywheel, which
is used for cooling and driving the centrifugal clutch, and a magnetic pulse that
controls the ignition system by magnets moulded into the flywheel.
The dimensions of the engine
compartment in the existing prototype require a compact design of the power
unit and a crankshaft speed sensor to initiate the run-up mechanism. The design
of the components must be focused on simplicity, reliability, low weight and
compact dimensions.
The appropriate solution for
transferring the torque of the engine and the starting mechanism involves a
pair of automatic centrifugal clutch. Questions concerning engine dynamics and related
problems are analysed in [3], [4].
To meet the competition rules, both automatic
clutches must be constructed to engage different speeds in order to prevent the
transmission of torque from the electric starter motor to the driven wheel via
the clutch and shaft. To comply with this rule, centrifugal clutch springs
for the wheel drive stiffen the coupling elements with a lower weight or a
bigger radius.
The difference in clutch engagement
speed is shown in Fig. 1.
Fig. 1. Clutch engagement diagram
The following calculates the centrifugal force
acting on the operating coupling elements:
(1)
The mass of the clutch element in
relation to the centrifugal force must be calculated according to the part that
is particularly affected. The weight of the mounting part of the element, which
moves around the guide pin, must not be included in any calculation.
The dimensions of the centrifugal
clutch elements for the torque transmission depend on the friction linings’
coefficient, their number, the friction radius, the centrifugal force and the
spring force, which keeps the coupling elements in the off position.
The following equations are for
calculating the linear spring stiffness and friction torque:
(2)
(3)
(4)
In order to save as much space as
possible when mounting the engine, three narrow clutch elements were
constructed. Torque and heat transfer of the three elements will be more
reliable in racing when using the start-stop power unit system to save fuel.
The course of the load in relation
to time when using the start-stop system, as commonly used in Shell Eco-marathon
races, is shown in Fig. 2.
Fig. 2. Engine load during races
3. DESIGN
To avoid losses in the starting
mechanism drive, the spur gear with mounted centrifugal clutch elements rotates
around the engine crankshaft on ball bearings, which are secured on the outer
bearing ring with screws. Mechanical losses involving the driving mechanism are
serious problems, as discussed in [5], [6]. The mechanical losses, which are
connected to mechanical vibrations, are usually considered as negative
phenomena [7]. With regard to simplicity, interchangeability of parts and
reliability, the same construction and number of friction elements for the drive
clutch are used. The difference between the starter and drive clutch
construction is stated in chapter 2.
The complete assembly cut-out of the
dual-purpose drive clutch without tension springs and cooling holes in the friction
wheels is shown in Fig. 3.
Fig. 3. Dual-purpose drive clutch cut-out:
(1) crankshaft, (2) starter spur gear, (3) hub, (4) toothed flywheel, (5)
centrifugal drive clutch friction wheel, (6) nut, (7) clutch element, (8) ball
bearing and (9) needle bearing
Each rotating disc in the dual
clutch must be precisely balanced and manufactured to avoid crankshaft and
bearing failure, which result from the small dimensions and design of the
original power unit.
The used hub, which is mounted on the
output side of the crankshaft with a key, creates more space, as well as offers
more options to use other parts, such as bearings, locking keys and surfaces.
To achieve the minimum inertia and weight in each part, the spur gear, engine
flywheel and output friction wheel are designed with holes on their faces. Heat
transfer and the cooling of friction clutches are important in terms of making
them reliable in the course of every race lap. The design of every race
component also aims for simplicity regarding the speed of changing and
repairing each part or the whole assembly. Simply put, easy access for screws,
nuts and locking rings must take place.
Fig. 4. 3D cut-out of a complete dual-purpose
clutch assembly
4. CONCLUSIONS
1.
One of the possible complex designs of a starting
and drive mechanism for small power units aimed to avoid the maximum possible
losses.
2.
This design was compromised by low part inertia,
reliability, simplicity and tough race conditions.
This
paper was elaborated within the framework of the following projects:
VEGA1/0197/14 – research on new methods and innovative design solutions in
order to increase efficiency and to reduce emissions of transport vehicle
driving units, together with the evaluation of possible operational risks; VEGA
1/0198/15 – research on innovative methods for emission reduction of driving
units used in transport vehicles and optimization of active logistic elements
in material flows in order to increase their technical level and reliability;
and KEGA 021TUKE–4/2015 – development
of cognitive activities focused on innovations in educational programmes in the
discipline of engineering, as well as building and modernizing specialized
laboratories specified for logistics and intra-operational transport.
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Received 09.12.2015; accepted in revised form 19.03.2016
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