elevator counterweight design and stress analysis

ELEVATOR COUNTERWEIGHT DESIGN AND
STRESS ANALYSIS
by
Ali Sinan ERTÜRK
September, 2008
İZMİR
ELEVATOR COUNTERWEIGHT DESIGN AND STRESS ANALYSIS
ABSTRACT
Elevator systems, seen as luxury items in the past, today compulsory in buildings five
stories and higher, are the fastest growing mechanisms of vertical transport sector in parallel
to technology. Elevator dimensions are entirely determined according to requirement, higher
capacity or two or more elevators are built when the number of people who will make use of
the elevator increases in a building. Capacity increase gives the materials to be used in its
construction more significance, those that are more durable, light and economic are preferred.
In this study, development of elevator systems until today are discussed first; afterwards,
elevator parts are introduced and briefly explained and finally, elevator counterweight design
studies and related analyses are carried out. Elevator counterweight is part of the system,
which enables its movement and balances the system. In the design studies, economic aspects,
as well as, counterweight strength are considered as the criteria for the optimum design.
In carrying out the studies, calculations have been done in order to determine the types and
intensities of loads the elevator counterweight will be subjected to. Software program
SolidWorks has been utilized for designing the counterweight and the analyses are carried out
in CosmosWorks, which operates in tandem with SolidWorks and based on FEM (Finite
Elements Method). Short introductions of softwares SolidWorks and CosmosWorks are also
included in this study, along with the reasons of why they have been employed.
In the final stage of the study, comments have been made on the results of the analyses, by
also taking into consideration the economic aspects, a new elevator counterweight design has
been proposed.
Keywords: Elevator Counterweight, Stress Analysis, Safety Gear, CosmosWorks
ASANSÖR KARŞI AĞIRLIK TASARIMI VE GERİLME ANALİZİ
ÖZ
Geçmişte lüks olarak görülen, günümüzde ise kanunen beş ve daha yüksek katlı binalarda
zorunlu olan asansör sistemleri, düşey transport sektörünün teknolojiyle paralel olarak en hızlı
gelişen mekanizmalardır. Asansör boyutu tamamen ihtiyaca göre belirlenmekte, binada
asansörü kullanacak insan sayısı arttıkça daha büyük kapasiteli ya da iki veya daha fazla
asansör yapılmaktadır. Kapasite arttıkça kullanılan malzemeler de önem kazanmakta, daha
mukavim, hafif ve en ekonomik olanları tercih edilmektedir.
Yapılan bu çalışmada, asansör sisteminin günümüze kadar nasıl geliştiği, asansör
parçalarının kullanım amaçları belirtilmiş olup, asansör sisteminin hareketini sağlayan,
sistemi dengeleme görevine sahip asansör karşı ağırlığı tasarımı ile ilgili çalışma ve analizler
yapılmıştır. Tasarım çalışmalarında, dayanıklılığın yanı sıra ekonomik boyut da incelenmiş
olup, kullanılabilecek en uygun karşı ağırlık tasarımı belirlenmeye çalışılmıştır.
Çalışmanın yürütülmesi sırasında belirli hesaplamalar yapılarak asansör karşı ağırlığının
hangi yüklere maruz kalabileceği belirlenmiştir. Tasarım için SolidWorks, yapılan
tasarımların analizleri için yine SolidWorks programıyla beraber çalışan FEM (Finite
Elements Method - Sonlu Elemanlar Metodu) yöntemini esas alan CosmosWorks
programlarından
yararlanılmıştır.
Çalışmada
ayrıca
SolidWorks
ve
CosmosWorks
programlarının kısa tanıtımları yapılarak analiz için bu programların seçilme nedenlerine
değinilmiştir.
Çalışmanın son bölümünde analiz sonuçları yorumlanmış, maliyet hesapları yapılıp
ekonomik boyut da göz önünde bulundurularak yeni bir karşı ağırlık tasarımı oluşturulmuştur.
Anahtar Sözcükler: Karşı Ağırlık, Gerilme Analizi, Mekanik Fren, CosmosWorks
1. Introduction
In 95/16/AT Elevator Regulations book, an elevator is defined as “lifting device consisting of
a platform or cage having more than 15 degrees with the horizontal, that is raised and
lowered mechanically in a vertical shaft by means of rails in certain limits in order to move
people or loads from one floor to another in a building”. Different standards are used for the
calculations of elevators having 15 degrees with the horizontal and the vertical. TS 10922 EN
81-1 and TS EN 81-2 standards refer to vertical elevators.
1.1 Impotrance of Counterweight in Elevator Systems
Elevators systems enable their motion by counterweights, also known as balance weights.
The total load on the counterweight side is computed by adding one half of the declared load
to the total load of the elevators car (Figure 1). This way, the elevator electric motor is
subjected to an unbalanced load only half of the declared laod when the elevator runs empty
or full.
An elevator counterweight comprises a counterweight frame constructed from several vertical
beams and at least three horizontal crossbars wherein the vertical beams penetrate the
horizontal crossbars and form therewith several grid fields in which weight elements are
arranged and fixed. The two outermost grid fields disposed above the lowermost horizontal
crossbar are open towards the side and can each receive a counterweight guide shoe and a
safety brake device. Various materials such as concrete or pig casting can be used as weights
(Tavaslıoğlu, 2005).
Mechanical brakes are generally applied to the car side. Today, double sided safety
systems are utilized, they are designed as preventing the motion of the empty car upwards and
full cabin downwards. But, TS 10922 EN 81-1 declares that if there is enough space beneath
the ground where the elevator system sits, the ground should be able to withstand at least
5000 N/m2 dynamic load and the counterweight should be equipped with a mechanical brake.
Figure 1 Elevator drive system
2. Modelling and Analysis
2.1 Counterweight Modelling and Analysis
In today’s ever developing world it is important that a product is delivered to the customer
as soon as possible. It is the same for elevator manufacturing, the products are manufactured
with the latest technology and they are to be asembled easily. Elevator counterweights are
started to be manufactured from sheet plates rather than NPU profiles (Figurel 2). This way,
production process and transportation have gained speed and aseembly failures have been
reduced to a minimum.
Figure 2 Elevator
counterweight frame
model
A simple counterweight frame made from sheet profile has been modelled by using the
SolidWorks software. Since the SolidWorks software runs based on parasolid principles it
enables the user to interfere at all stages of design, thereby, enabling to change the dimensions
and details of the model and assembling the parts.
Stress analyses are carried out on CosmosWorks, which runs under SolidWorks software.
CosmosWorks software, like other software based on finite elements method (Catia,
Unigraphics, ProEngineer…), enables to obtain structural analysis results on solid models by
giving the boundary conditions and loads as inputs. The systems presents the results to the
user in an exremely user friendly way. This way, data loss and faluty data reading are
prevented since modelling and analysis run within the same system (Bayrak and Turgut,
2008).
Elevator counterweight analyses should be carried out in two stages, namely, the stresses
and dispalcements arising from the normal usage of the counterweight and
when the
mechanical brake is in use. In order to proceed with this examination, it is necessary to carry
out stress analyses on the counterweight frame for both stages.
2.2 Counterweight model analysis – normal usage:
Counterweight model constructed from 5 mm thick sheet profiles, has a weight of 8436,6
N including the weight put into it. The material has been selected as pure carbon steel which
has mechanical properties very similar to St 37 structural steel (Tables 1 and 2). Prior to
analysis, the counterweight bas eis subjected to a load of 5886 N. The frame’s own weight has
been aasigned automatically using the accelaration of gravity and it is fixed assuming that it is
hanged from the top by elevator ropes (Figure 3).
Table 1 Mechanical properties of St37 material (Yeni, 1998)
Material:
Property
Modulus of Elasticity
Tensile Strength
St37
Value
2.1e+011
3.68e+008
Unit
N/m2
N/m2
Table 2 Mechanical/physical properties of pure carbon steel
Material:
Property
Modulus of Elasticity
Poisson Ratio
Sheer Modulus
Specific Weight
Tensile Strength
Yield Strength
Thermal Expansion Coef.
Thermal Permeability
Pure Carbon Steel
Value
Unit
2.1e+011
N/m2
0.28
7.9e+010
N/m2
7800
kg/m3
3.9983e+008 N/m2
2.2059e+008 N/m2
1.3e-005
/Kelvin
43
W/(m.K)
Figure 3 Support point,
load application location
The stresses, displacements and design check results obtained from the counterweight
frame model analysis are given in Figures 4, 5 and 6. The results obtained are presented with
a deformation ratio of 1360/1.
Figure 4 Counterweight model – normal usage – stress analysis results
Figure 5 Counterweight model – normal usage – displacement analysis
results
Figure 6 Counterweight model – normal usage – design check results
The analysis results how that during normal usage the maximum stress occuring on the
counterweight frame is 2,956x107 N/m2. The location of this maximum value is on the holes
where it is assumed that the rope connections are made. Taking into consideration the yield
strength of the material, the model is 7,5 times strong. This value can be seen in design check
results. Maksimum displacement occurs on the vertical sides of the frame as 2,829x10-4 .
2.3 Counterweight model analysis – mechanical brake in operation:
When a mechanical brake is added to the counterweight model, the value of the load acting
on the counterweight should be calculated in order to investigate the stresses and
displacements occuring during braking.
According to TS 10922 EN 81-1 App. F clause F.3.3.3.1, the braking force of a double
direction mechanical brake is calculated as,
(P + Q ) = BrakingForce
16
Which gives;
P = 710 kg
Q = 300 kg
(P + Q ) = BrakingForce ⇒ (710 + 300) = BrakingForce
16
16
Braking force =16160 kg. 9,81 =158529,6 N
On the counterweight frame model shown in Figure 2, there are four bolt holes on each
vertical side. The braking force calculated is the total load, therefore the load that will act on
each bolt hole will be 1/8 of this value.
Load acting on each bolt hole = 158529,6 / 8 = 19816,2 N
The stresses, displacements and design check results obtained from the are given in Figures
7, 8 and 9. The results obtained are presented with a deformation ratio of 280/1.
Figure 7 Counterweight model – mechanical brake in use – stress analysis results
Figure 8 Counterweight model – mechanical brake in use – displacement analysis
results
Figure 9 Counterweight model – mechanical brake in use – design check results
When the mechanical brake is in use the maximum stress value on the counterweight frame
model is obtained as 1,546x108 N/m2 , this value is close to the yield strength of the material.
The ratio is about 1,4/1, this is alos the design check result. This shows that, the model is 1,4
times strong. The displacement value is 1,247x10-3 m.
2.4 Modification of the counterweight model:
As a reslut of the analyses carried out, the maximum stresses occuring on the
counterweight frame are below the yirld dtrength of the material. On the other side, there is a
possibilty to enhance the analysis results by increasing the thickness of the sheet material used
in constructing the counterweight model, thereby modification studies have been carried out.
In this study, the thickness of the sheet material has been increased from 5 mm to 6 and 8
mm, respectively. In this case, the sytem certainly will become stronger. The important point
is how many times the product will become stronger.
The stress and design check results when the mechanical brake is in use for a
counterweight model made of 6 mm thick sheet material are given in Figures 10, 11, 12 and
13. The results presented in Figures 14, 15, 16 and 17 belong to the analysis results of a
counterweight model made of 8 mm thick sheet material.
Figure 10 Counterweight model – 6 mm thickness – normal usage – stress
analysis results
Figure 11 Counterweight model – 6 mm thickness – normal usage – design
check results
Figure 12 Counterweight model – 6 mm thickness - mechanical brake in use –
stress analysis results
Figure 13 Counterweight model – 6 mm thickness - mechanical brake in use –
design check results
The results obtained from incerasing the counterwegiht sheet profile thickness to 6 mm
reveal that the maximum stress value is 2,547x107 N/mm2. Design check results show that the
model is 8,67 times strong. Maximum displacement value is 2,449x10-4.
The maximum stress value when the mechanical brake is in use is 1,243x108 N/mm2 .
Maximum displacement value is 1,172x10-3 mm and design check results give that the model
is 1,775 times strong.
Figure 14 Counterweight model – 8 mm thickness – normal usage – stress
analysis results
Figure 15 Counterweight model – 8 mm thickness – normal usage – design
check results
Figure 16 Counterweight model – 8 mm thickness - mechanical brake in use –
stress analysis results
Figure 17 Counterweight model – 8 mm thickness - mechanical brake in use –
design check results
When the counterweight model is produced from 8 mm thick sheet profiles, the value of
the maximum stress in mormal usage is 2,913x107 N/mm2 . Maksimum displacement value is
1,893x10-4 mm, design check results show that the model is 7,6/1 times strong.
The maximum stress when the mechanical brake is in use is 8,92x107 N/mm2 and design
check results reveal that the model is 2,5 times strong. Maksimum displacement, in this case,
is 7,156x10-4 mm.
2.5 Cost analysis of counterweight:
Several analyses have been carried out in an elevator counterweight design in order to
obtain the product possesing the best stress values and most convenient to use. But, besides of
a product’s mechanical properties, its manufacturing cost should also be taken into
consideration. Low cost and high strength will be a convenient and strong one indeed.
Therefore, it is a precondition to carry out the cost analyses of the counterweight models
being constructed. Table 3 presents the manufacturing costs of the three different thickness
models constructed.
Table 3 Manufacturing costs of the three different thickness models
Model Name
Counterweight model – Sheet
Thickness 5 mm
Counterweight model – Sheet
Thickness 6 mm
Counterweight model – Sheet
Thickness 8 mm
Cost
Ratio
280,00 YTL
1
330,00 YTL
~1,17
420,00 YTL
~1.5
When the given costs are taken into consideration and assuming that the product is suitable
for mass production, the second model should be selected. Because, if the cost differences are
examined, it is seen that between models one and two there is only 50,00 YTL difference,
while between model one and three there is a much higher difference of 140,00 YTL.
With these costs, the most suitable counterweigth model is the one with the sheet thickness
of 6 mm.
3 Results and Discussion
The comparison of analysis results carried out for the counterweight model made from
sheet material are given in Tables 4 and 5.
Table 4 Results obtained from analyses– Normal usage
Normal Usage
Counterweight model – 5 mm
sheet thickness
Counterweight model – 6 mm
sheet thickness
Counterweight model – 8 mm
sheet thickness
Maximum Stress
Value [N/mm2]
Design
Check
Results
2,956x107
7,5
2,547x107
8,67
2,913x107
7,6
As seen in Table 4, among the models, the lowest stress value is obtained for the second
model with the sheet thickness of 6 mm. Increasing the sheet thickness lowers the stress
values but increases the product’s weight. This brings difficulty of assembly of the product.
But it is seen that, in the model with 8 mm of sheet thickness contrary to the expectations that
the stress values will decrease, some increase has seen in the stress values.
Tablo 5 Results obtained from analyses – Mekanical brake in use
Mechanical Brake in Use
Counterweight model – 5 mm
sheet thickness
Counterweight model – 6 mm
sheet thickness
Counterweight model – 8 mm
sheet thickness
Maximum Stress
Value [N/mm2]
Design
Check
Results
1,546x108
1,4
1,243x108
1,775
8,92x107
2,5
Examination of Table 5 reveals that, lowest stress and displacement values obtained from
the analyses results belong to the model with 8 mm sheet thickness. It is kown that this same
model have not yielded the best results for normal usage. Therefore, this model is not suitable
for use for both types of usage. In this case, the second model having the lowest values should
be examined. 6 mm thickness model, which has yielded the best results for normal usage
seems to be the strongest model against the forces which occur when the mechanical brake is
in use.
In this study, with the aid of an anlysis software based on computer aided design and finite
elemensts method, the most suitable product is aimed to obtain by carrying out several
analyses.
As a result, taking into consideration the cost analyses of the counterweight models, the
model having a sheet thickness of 6 mm appears to be the best model for both normal usage
and when the mechanial brake is in use.
Acknowledgements
I would like to thank my supervisor Assist. Prof. Dr. Çınar Yeni for her valuable help and
advice.
I am also indepted to my family and Yasemin Türese for their endless patience, support
and love.
References
Bayrak, Sevilay ve Turgut, Mustafa (2008), SolidWorks, CosmosWorks, CosmosMotion,
MoldFlow, SolidCam (2. Baskı), Ankara: Seçkin Yayıncılık
Tavaslıoğlu, Serdar (2005), Asansör Uygulamaları (2. baskı). İzmir: Final Matbaacılık ve
Ticaret
Türk Standardı (2001), TS 10922 EN 81-1 Asansörler – Yapım ve Montaj İçin Güvenlik
Kuralları – Bölüm 1: Elektrikli Asansörler. Ankara: Türk Standartları Enstitüsü
Yeni, Çınar E. (1998), Strength Mis-Match Effect On Fracture Behaviour Of Structural Steel
Welds, İzmir: Dokuz Eylül Üniversitesi Fen Bilimleri Enstitüsü Doktora Tez Arşivi