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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: St37

Property Value Unit Modulus of Elasticity 2.1e+011 N/m2

Tensile Strength 3.68e+008 N/m2

Table 2 Mechanical/physical properties of pure carbon steel

Material: Pure Carbon Steel

Property Value Unit Modulus of Elasticity 2.1e+011 N/m2

Poisson Ratio 0.28 Sheer Modulus 7.9e+010 N/m2

Specific Weight 7800 kg/m3

Tensile Strength 3.9983e+008 N/m2

Yield Strength 2.2059e+008 N/m2

Thermal Expansion Coef. 1.3e-005 /Kelvin Thermal Permeability 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,

( )16

ceBrakingForQP =+

Which gives;

P = 710 kg

Q = 300 kg

( ) ( )16

30071016

ceBrakingForceBrakingForQP =+⇒=+

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


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


stress analysis results


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 Cost Ratio

Counterweight model – Sheet

Thickness 5 mm 280,00 YTL 1


Thickness 6 mm 330,00 YTL ~1,17


Thickness 8 mm 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 Maximum Stress

Value [N/mm2]

Design

Check

Results

Counterweight model – 5 mm

sheet thickness 2,956x107 7,5





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 Maximum Stress

Value [N/mm2]

Design

Check

Results







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

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