Article citation information:
Medvecká-Beňová, S. Strength analysis of the
frame of a trailer. Scientific Journal of
Silesian University of Technology. Series Transport. 2017, 96, 105-113. ISSN: 0209-3324. DOI: https://doi.org/10.20858/sjsutst.2017.96.10.
Silvia MEDVECKÁ-BEŇOVÁ[1]
STRENGTH ANALYSIS OF THE FRAME
OF A TRAILER
Summary. A
trailer for carrying a small boat or water scooters was designed for a private
sector company. It was a category O1 non-braked trailer with a gross weight up
to 750 kg and a height-adjustable traction device. The frame must be designed
and installed in such a way that, during its proposed lifetime, it will
withstand all the loads and influences that may occur during construction and
operation with an appropriate level of reliability. The designed frame of the
trailer is a welded galvanized structure. The article is devoted to the
problems related to the stress analysis of a trailer frame.
Keywords: trailer; frame; design; strength
analysis; finite element method
1. INTRODUCTION
The design of a frame structure is based on the
knowledge and experience that are commonly available at the time the design is
proposed [7-8, 10, 11]. The supporting structure (frame) must be designed and
installed in such a way that, during its proposed lifetime, it will withstand
all the loads and influences that may occur during construction and operation
with an appropriate level of reliability. Furthermore, the designed support
(frame) structure must meet the relevant user requirements specified for the
support structure or supporting element. The support (frame) structure must be
designed to have the appropriate durability and usability.
Possible damage to the steel structure (frame)
must be eliminated or prevented by the appropriate choice of measures [6]. These include ensuring the integrity of the load-bearing
structure and selecting a support structure that is considered to be capable of
carrying an unforeseen loss of an individual element or enduring acceptable
local damage.
It is also important to take appropriate
diagnostics during operationalization into account, for example, by
non-invasive methods [1-4, 9].
2. trailer
for transporting boats and personal watercraft
A trailer was designed for a private
sector company. It was a non-braked trailer (category O1, gross weight up to
750 kg) with a height-adjustable traction device for carrying small boat or
water scooters. The designed trailer and its accessories are shown in Fig. 1.
The support (frame) structure must
be designed to have the appropriate durability and usability.
Fig. 1. The designed trailer with
accessories
The descriptors for Fig. 1 are as
follows: 1) a cross beam; 2) longitudinal beam; 3) guide rollers; 4) a draw-out
light panel; 5) supporting leg with holder; 6) wheel; 7) axletree; 8) guide skis; 9) height-adjustable lift device; 10) supporting
wheel with holder; 11) height-adjustable reel; 12) connecting hinge.
The axle (position 7 in Fig. 1) is
mounted under the frame and fulfils the function of the intermediate transverse
beam. Between the axle (position 7 in Fig. 1) and the trailer frame are two
cross beams (position 1 in Fig. 1), which reduce stress and deformation on the
frame of the trailer. When loading and unloading a boat or water scooter, it is
necessary to immerse the trailer in the water. The trailer truck is designed to
have a total weight of not more than 750 kg. This means that the maximum
weight can be 450 kg because the weight of the separate trailer is 240 kg.
An important component is the boat
carrier for the transport of boats. It is attached to the frame by means of a
boulder clamp. The designed holder for the guide rail is shown in Fig. 2.
Position 1 is the top of the guide rail, which comprises 4 mm of thick sheet
metal. The centre holder of the cam chain guide is in
position 2. The bottom holder of the cam chain guide (position 3) also
comprises 4 mm of thick sheet metal. In position 4 is a screw with a washer, while a boat carrier is located in
position 5.
Fig. 2. The designed holder for the guide
rail
3. Design
of a trailer frame
The designed frame of the trailer is
a welded galvanized structure. The frame consists of two longitudinal beams
(position 1), two cross beams (position 2) and one shorter cross beam (position
3), as shown in Fig. 3.
These elements are joined together
by welded joints. The overall frame dimensions are 5,200
mm x 1,950mm. The longitudinal beams (position 1 of Fig. 3) are made of the profile 80 x 40 x 4 mm (h x b x t in Fig. 4).
The cross beams (position 2 of Fig. 3) are made of
the profile 50 x 50 x 4 mm (5 x b x t in Fig. 4) and the shorter cross
beam (position 3 of Fig. 3) is made of the profile 80 x
40 x 4 mm (h x b x t in Fig. 4).
All parts of
the frame are made of material W. Nr. 1.0039. This material has guaranteed
weldability.
Fig. 3. The frame of the designed
trailer
Fig. 4. The profile of the frame beams
4. Calculation
of fillet weld according to STN EN 1993-1-8
Welds are checked to connect the
cross beams (position 2 of Fig. 3) with the profile of the frame of the
trailer. The fillet weld is made around the circumference. The size of the filled weld is a = 2.1 mm (z = 3 mm). According
to STN EN 1993-1-8, the design resistance of the fillet weld is checked. Design
resistance to fillet welding is sufficient when conditions (1) and (2) are met.
(1)
(2)
where:
– normal stress perpendicular to the plane of the weld [MPa]
– shear stress (in the plane of welding) parallel to the
welding axis [MPa]
– shear stress (in the plane of welding) perpendicular to the
welding axis [MPa]
– nominal tensile
strength of the weaker of the joined parts [MPa]
– partial reliability factor [-]
– relevant collector factor [-]
The
calculation follows:
Since the two conditions are met,
the weld is compliant.
To compare the results, this weld
was also checked using the standard calculation procedure according to the
following equation:
(3)
where:
– result stress in the
weld [MPa]
– shear stress (in the plane of welding) parallel to the
welding axis [MPa]
– shear stress (in the plane of welding) perpendicular to the
welding axis [MPa]
– welding factor for the
given load direction [-]
– welding factor for the given load direction
[-]
– yield stress [MPa]
– coefficient of the thickness of the filled weld [-]
k – coefficient of security measure [-]
The
calculation follows:
Since the two conditions are met,
the weld is compliant.
5. Strength
analysis of the frame by FEM
Recently, given ever-faster evolving computer technology and the
available literature, we can encounter modern numerical methods, such as the
finite element method (FEM) [5, 13]. It is one of the most widely used numerical
mathematical methods for solving the problems of elasticity and strength, the
dynamics of pliable bodies, heat transfer, fluid flow, electromagnetism, and
many other problems in engineering.
An
important part is the definition of boundary conditions. The three-dimensional
model of the trailer was divided into four parts in order to create the
so-called nodule point. Nodule points serve as load points for load definition.
The second part of the longitudinal beam represents the start and end of the
axle of the trailer. At these two nodule points, binding was defined using the
“fixed geometry” function, which prevented movement in every direction. The cross beams were divided into three sections. The nodule
points on the cross beams make up the places where the guide skates are held.
At the nodule points of the cross beams, the binding was defined using the
“external loads” function, which represents the total load force applied to the
cross beams. The wearing force acting on the cross beams had to be calculated.
Load force is calculated from the weight of:
- the transported boat (450 kg)
- the cam chain guide (position 8 of Fig. 1)
- upper holder of the cam chain guide (position 1 of Fig.
2)
- centre holder of the cam chain guide (position 2 of Fig.
2)
- screw and self-locking nut
- bottom holder of the cam chain guide (position 2 of Fig.
2)
The total load on the frame is determined by the calculation FC =
4,645 N. Stress analysis by the FEM of the frame of the designed trailer is
shown in Fig. 5.
Fig. 5. Trailer frame stress
(reduced stress according to HMH strength theory)
The maximum stress is located in the points of
attachment of the axle to the trailer frame. The maximum stress achieved has a
value σmax = 118.8 MPa. The maximum stress value does not
exceed the permitted stress of σD = 210MPa. The results show
that the designed frame of the trailer meets the strength conditions.
6. FRAME DEFLECTION
The deflection of the trailer frame by the FEM is
showed in Fig. 6. The maximum deflection occurred at the front of the trailer
frame; its size is 7.67 mm. In practice, the deflection is much smaller at this
point because the front part of the trailer is coupled with the trailer drawbar
by the welded joint.
Fig. 6. Deflection of the trailer
frame
After loading the boat or water scooter, most of the
cross beams are loaded. The deflection in the centre of the cross beams is 2.84
mm (Fig. 6). When designing similar types of trailers for the transport of
boats or water scooters, the greatest attention must be paid to the cross beam.
These elements carry most of the load.
7. CONCLUSION
The frame of the trailer must be designed to have the
appropriate durability and usability. The design of the frame is based on the
knowledge and experience that are commonly available at the time the design is
proposed. The designed trailer frame is designed as a welded structure. In our
case, all welds were by checked for strength according to STN EN 1993-1-8 [12].
Stress analysis of trailer frame was made using the FEM. When designing similar
types of trailers for the transport of boats or water scooters, the greatest
attention must be paid to the cross beam. These elements
carry most of the load. The results obtained show that the designed
trailer frame meets the strength and deformation conditions and can fulfil its
function.
Acknowledgements
This paper was written within the framework of the
VEGA grant project, “1/0473/17: Research and development of technology for
homogeneous charge self-ignition using compression in order to increase engine
efficiency and to reduce vehicle emissions”.
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Received 02.05.2017; accepted in revised form 30.07.2017
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