Stability in Furniture

Introduction

Furniture has become our basic need which supports our activities like seating, working, storing, sleeping and even a decorative art to the living room. Several standards are formatted and been followed across the globe for safety & performance of the product. Tipping is one of the mandatory tests to pass through to sell the product across the global. This test should be performed with extreme care as this will have a high impact on the consumer. The furniture failing on other performance tests will onlylead to warranty claims, but the failure in tipping test shall lead to series injuries including fatal deaths to the consumer.

We at ATIPL Altran Technologies Limited have very good experience in handling furniture design including tipping / stability. We also provide suitable design suggestion to avoid tipping without deterioration in the performance. We provide design solutions in furniture to comply with standards like ANSI/BIFMA and UL standards. We also provide design solution for customer specific standards for various furniture like educational, office, lounge chairs & tables, storage equipment’s, draw chests., etc.

Importance of Stability

Serious injuries which are leading to fatal deaths are happening for decades together all around the world. In most of the cases, the accidents are not reported.

Still in many countries there are no standards / regulations to avoid these accidents. These accidents are not only happening to children, it happens to adults and even to seniors. According to U.S Consumer Product Safety Commission report which is published on August 2014, about 1,92,900 were treated for injuries from 2006 to 2013 only due to Furniture[1].
j1

Fig1: A Child climbing a drawer [2]

Note: The number shown in square brackets indicates the references used, which are shown in reference section.

In 2011, 49 children were killed and couple of thousands were taken to hospital with series injuries [3]. A two-year-old toddler was trapped to death when a chest tipped on him, this year. This resulted a furniture giant to recall 35 million products sold all over the world since 2002 [4].

The general solution for the tipping would be anchoring the unit to the wall as shown below, but all the furniture’s designed can not be anchored to avoid tipping. We suggest a apt solution to avoid tipping from FEA and Analytical approach. Knowing the criticality effect of tipping of furniture, we focus on this failure mode even before a design is freezed

j2
Fig 2: Anchor Method [5]

Methods to overcome Stability

Floor Standing Unit – Portable

The tipping will happen easily when the height of the furniture is more than its width and depth, this can be avoided by fixing it to wall. But not every furniture can be attached to a wall. This is a Portable wall standing storing unit (fig 3) which is designed to be fixed for tipping.

j3
Fig 3: FE model of Floor standing unit

The UL standard states that a wall standing unit will not tip when it is tilted to 10˚ when filled with maximum loading capacity and it’s doors closed”. We calculated the Centre of mass of the unit at the tilted condition, and based on the units’ pivot point (roller axis) the units’ stability can be calculated from the momentum equation.

The stability of the unit is calculated using hand calculation using the force and momentum equation, which is also matching with simulation using Ansys and Ls-Dyna. So to reduce the time taken in solving, we made template to arrive at the minimum dimension of the base design to avoid tipping.

When the Cg falls outside the pivot point on tilting, the other support lift off from the floor. Hence we suggested an optimized base design that will not tip for 10˚and made the minimum tipping angle to be 14˚. For the immobile storage unit model, we suggested to have the unit anchored to the wall to avoid tipping. Also the tipping can be avoided by lifting the front foot/levelers to certain height than the rear one. The modified base design model is shown in Fig.4

Capture1
Fig 4: Modified base design of floor standing unit

Table and Chair

Table is the most common furniture used worldwide. It fills nearly every home, office, lounge and multiple areas. Based on different application on table, there are set of standards to define the tipping load for table. The load will be applied to the least stable position of the table. Also proper constrains applied in the legs of the table to get the exact tipping load.

When the table is placed on the floor it will create a reaction force exerted on the table, that reaction force is tracked to observe the stability of the table. The below table design shows the possibilities of tipping before reaching the load as per the BIFMA standard of 125lbs at the user area.

The table is modeled in Ansys and proper constrains are provided to the table. The load is applied to the top of the table. The simulation setup is very much similar to match the exact behavior in the real testing.

j5
Fig. 5 Table Design

The above table model started tipping at 110 lbf load which is below the standard load of 125 lbf which is shown in Fig.6.

Capture2
Fig.6 Reaction force vs. Load

The table had complications like the design correction affecting the product aesthetics , hence mass addition will help the product to comply the test. The design suggestion as shown below (Fig.7) improved the tipping load of the table and also added new feature to the product.

j7
Fig.7 Modified Table Design with counter mass

After adding the counter mass in the form of steel plate to the other side of the table as shown in the above image, the tipping load increased from 110lbs to 140lbs which is safe for testing. Also the added mass is in the form of an additional feature to the product by providing support for the power cables which will be used by the electrical products. The same methodology is used to address the front and rear stability issue in different types of office and educational chairs according to BIFMA x5.1 and x6.1 standards.

Drawer Chest

Drawer chest (Fig.8) are majorly used for storing clothes, placing TV with decorative collection of toys and can be placed anywhere in the house. This furniture should be taken care specifically for its stability. Since these draws will be misused by the children to climb the chest. If these draws are not properly designed within the safe limits to avoid toggle, it will tip over making serious and fatal injurious to the children.

j8
Fig. 8 Drawer Chest

The tipping load of the drawer chest is determined by the FEA methods. Also will suggest ways to improve the stability of the drawer. Below shown image is one physical testing of drawer chest with child manikin (Fig. 9). The same condition is simulated and the tipping load for the draws is predicted. The tipping criteria will be slightly varied based on the requirement and the model. The FEA model of Child manikin (Fig. 10) is used and placed in the maximum moment creating position and dynamic tipping behavior is simulated.

j9
Fig.9 Chest Draw with Child manikin – Physical Testing [6]

j10
Fig.10 Chest Draw with Child manikin – FEA model test

Another possibility for the drawer to tip is due to shifting of its center of gravity towards the user during the draw open condition. Stability calculation will be made based on the worst case criteria according to the standard considering the maximum capacity of mass to be filled in the drawer. The change in the Centre of Gravity of the drawer is monitored and noted for its stability when draw is opened one by one (Fig.11)

Capture3

Fig.11 CG shifting in Drawer Table

The tipping in draw happens when the overhanging force creates a moment to shift the center of gravity to its least stable condition. Generally the draw tipping can also be avoided by using hinges fixed to the wall, but an optimized design will always speak the product safety and quality. Optimized model using parametric modeling combining with Ansys – APDL automation is created to achieve the proper support position with respect to the size of the draw. Using this automation approach, the design variables are achieved during the conceptual stage before entering into original design. Also using this automation approach, tipping of the draw with respect to number of draw in open condition can be determined.

In several cases the draws were failing and they needed to be fixed. We fixed all the stability problems by providing design suggestions and proved that these draws will pass for the stability criteria. Also we do not stick on only to the standard tipping cases as specified in the standard, we prefer and suggest some additional load cases to perform stability at the worst case when-ever we feel that it is going to fail which will be purely based on our experience.

Analytical Method for Stability

The analytical method can also be performed to observe the stability of the object. The analytical method uses the Force and Moment calculation to attain the equilibrium condition.

The simulation is simplified by converting the problem into simple free body diagram representing the forces, moments and constraints acting on the body. The free body diagram is shown below for the table shown in fig no.13.

j13

Fig. 12 Table Loading                    Fig.13-Free Body Diagram [7]

All the bodies will be considered as rigid bodies and converted into beams for simplification. The beam will be considered as simply supported beams and its force reaction will be observed to determine the stability of the object.

Summary

Tipping is a critical load case in furniture industry, as it is related to customer safety. Around the world lot of preventive measures and standards are evolving, creating an awareness within the people and forcing the manufacturer to design and develop a tipping free products.

References:

[1]. http://www.cpsc.gov//PageFiles/171154/InstabilityorTipoverReport2014Stamped.pdf

[2]. http://victoriachiropractor.ca/your-symptoms/

[3]. http://www.dailymail.co.uk/news/article-2934936/Distraught-parents-young-children-killed-falling-items-furniture-TV-sets-rally-raise-awareness-tip-deaths.html

[4]. http://www.ndtv.com/world-news/ikea-recalls-millions-of-dressers-in-us-canada-after-child-deaths-1425809

[5]. http://www.anchorit.gov/

[6]. https://easycpr-denver.com/wp-content/uploads/2015/09/Falling-TV-copy.jpg

[7]. Joseph E Shigley, Charles R Mischke, Richard G Budynas, Keith J Nisbett “Mechanical Engineering Design” – Eighth Edition Tata McGraw Hill Education Pvt., Ltd., pp 68-71.

Stability in Furniture

Introduction

Furniture has become our basic need which supports our activities like seating, working, storing, sleeping and even a decorative art to the living room. Several standards are formatted and been followed across the globe for safety & performance of the product. Tipping is one of the mandatory tests to pass through to sell the product across the global. This test should be performed with extreme care as this will have a high impact on the consumer. The furniture failing on other performance tests will onlylead to warranty claims, but the failure in tipping test shall lead to series injuries including fatal deaths to the consumer.

We at ATIPL Altran Technologies Limited have very good experience in handling furniture design including tipping / stability. We also provide suitable design suggestion to avoid tipping without deterioration in the performance. We provide design solutions in furniture to comply with standards like ANSI/BIFMA and UL standards. We also provide design solution for customer specific standards for various furniture like educational, office, lounge chairs & tables, storage equipment’s, draw chests., etc.

Importance of Stability

Serious injuries which are leading to fatal deaths are happening for decades together all around the world. In most of the cases, the accidents are not reported.

Still in many countries there are no standards / regulations to avoid these accidents. These accidents are not only happening to children, it happens to adults and even to seniors. According to U.S Consumer Product Safety Commission report which is published on August 2014, about 1,92,900 were treated for injuries from 2006 to 2013 only due to Furniture[1]. j1

Fig1: A Child climbing a drawer [2]

Note: The number shown in square brackets indicates the references used, which are shown in reference section.

In 2011, 49 children were killed and couple of thousands were taken to hospital with series injuries [3]. A two-year-old toddler was trapped to death when a chest tipped on him, this year. This resulted a furniture giant to recall 35 million products sold all over the world since 2002 [4].

The general solution for the tipping would be anchoring the unit to the wall as shown below, but all the furniture’s designed can not be anchored to avoid tipping. We suggest a apt solution to avoid tipping from FEA and Analytical approach. Knowing the criticality effect of tipping of furniture, we focus on this failure mode even before a design is freezed

j2 Fig 2: Anchor Method [5]

Methods to overcome Stability

Floor Standing Unit – Portable

The tipping will happen easily when the height of the furniture is more than its width and depth, this can be avoided by fixing it to wall. But not every furniture can be attached to a wall. This is a Portable wall standing storing unit (fig 3) which is designed to be fixed for tipping.

j3 Fig 3: FE model of Floor standing unit

The UL standard states that a wall standing unit will not tip when it is tilted to 10˚ when filled with maximum loading capacity and it’s doors closed”. We calculated the Centre of mass of the unit at the tilted condition, and based on the units’ pivot point (roller axis) the units’ stability can be calculated from the momentum equation.

The stability of the unit is calculated using hand calculation using the force and momentum equation, which is also matching with simulation using Ansys and Ls-Dyna. So to reduce the time taken in solving, we made template to arrive at the minimum dimension of the base design to avoid tipping.

When the Cg falls outside the pivot point on tilting, the other support lift off from the floor. Hence we suggested an optimized base design that will not tip for 10˚and made the minimum tipping angle to be 14˚. For the immobile storage unit model, we suggested to have the unit anchored to the wall to avoid tipping. Also the tipping can be avoided by lifting the front foot/levelers to certain height than the rear one. The modified base design model is shown in Fig.4

Capture1 Fig 4: Modified base design of floor standing unit

Table and Chair

Table is the most common furniture used worldwide. It fills nearly every home, office, lounge and multiple areas. Based on different application on table, there are set of standards to define the tipping load for table. The load will be applied to the least stable position of the table. Also proper constrains applied in the legs of the table to get the exact tipping load.

When the table is placed on the floor it will create a reaction force exerted on the table, that reaction force is tracked to observe the stability of the table. The below table design shows the possibilities of tipping before reaching the load as per the BIFMA standard of 125lbs at the user area.

The table is modeled in Ansys and proper constrains are provided to the table. The load is applied to the top of the table. The simulation setup is very much similar to match the exact behavior in the real testing.

j5 Fig. 5 Table Design

The above table model started tipping at 110 lbf load which is below the standard load of 125 lbf which is shown in Fig.6.

Capture2 Fig.6 Reaction force vs. Load

The table had complications like the design correction affecting the product aesthetics , hence mass addition will help the product to comply the test. The design suggestion as shown below (Fig.7) improved the tipping load of the table and also added new feature to the product.

j7 Fig.7 Modified Table Design with counter mass

After adding the counter mass in the form of steel plate to the other side of the table as shown in the above image, the tipping load increased from 110lbs to 140lbs which is safe for testing. Also the added mass is in the form of an additional feature to the product by providing support for the power cables which will be used by the electrical products. The same methodology is used to address the front and rear stability issue in different types of office and educational chairs according to BIFMA x5.1 and x6.1 standards.

Drawer Chest

Drawer chest (Fig.8) are majorly used for storing clothes, placing TV with decorative collection of toys and can be placed anywhere in the house. This furniture should be taken care specifically for its stability. Since these draws will be misused by the children to climb the chest. If these draws are not properly designed within the safe limits to avoid toggle, it will tip over making serious and fatal injurious to the children.

j8 Fig. 8 Drawer Chest

The tipping load of the drawer chest is determined by the FEA methods. Also will suggest ways to improve the stability of the drawer. Below shown image is one physical testing of drawer chest with child manikin (Fig. 9). The same condition is simulated and the tipping load for the draws is predicted. The tipping criteria will be slightly varied based on the requirement and the model. The FEA model of Child manikin (Fig. 10) is used and placed in the maximum moment creating position and dynamic tipping behavior is simulated.

j9 Fig.9 Chest Draw with Child manikin – Physical Testing [6]

j10 Fig.10 Chest Draw with Child manikin – FEA model test

Another possibility for the drawer to tip is due to shifting of its center of gravity towards the user during the draw open condition. Stability calculation will be made based on the worst case criteria according to the standard considering the maximum capacity of mass to be filled in the drawer. The change in the Centre of Gravity of the drawer is monitored and noted for its stability when draw is opened one by one (Fig.11)

Capture3

Fig.11 CG shifting in Drawer Table

The tipping in draw happens when the overhanging force creates a moment to shift the center of gravity to its least stable condition. Generally the draw tipping can also be avoided by using hinges fixed to the wall, but an optimized design will always speak the product safety and quality. Optimized model using parametric modeling combining with Ansys – APDL automation is created to achieve the proper support position with respect to the size of the draw. Using this automation approach, the design variables are achieved during the conceptual stage before entering into original design. Also using this automation approach, tipping of the draw with respect to number of draw in open condition can be determined.

In several cases the draws were failing and they needed to be fixed. We fixed all the stability problems by providing design suggestions and proved that these draws will pass for the stability criteria. Also we do not stick on only to the standard tipping cases as specified in the standard, we prefer and suggest some additional load cases to perform stability at the worst case when-ever we feel that it is going to fail which will be purely based on our experience.

Analytical Method for Stability

The analytical method can also be performed to observe the stability of the object. The analytical method uses the Force and Moment calculation to attain the equilibrium condition.

The simulation is simplified by converting the problem into simple free body diagram representing the forces, moments and constraints acting on the body. The free body diagram is shown below for the table shown in fig no.13.

j13

Fig. 12 Table Loading                    Fig.13-Free Body Diagram [7]

All the bodies will be considered as rigid bodies and converted into beams for simplification. The beam will be considered as simply supported beams and its force reaction will be observed to determine the stability of the object.

Summary

Tipping is a critical load case in furniture industry, as it is related to customer safety. Around the world lot of preventive measures and standards are evolving, creating an awareness within the people and forcing the manufacturer to design and develop a tipping free products.

References:

[1]. http://www.cpsc.gov//PageFiles/171154/InstabilityorTipoverReport2014Stamped.pdf

[2]. http://victoriachiropractor.ca/your-symptoms/

[3]. http://www.dailymail.co.uk/news/article-2934936/Distraught-parents-young-children-killed-falling-items-furniture-TV-sets-rally-raise-awareness-tip-deaths.html

[4]. http://www.ndtv.com/world-news/ikea-recalls-millions-of-dressers-in-us-canada-after-child-deaths-1425809

[5]. http://www.anchorit.gov/

[6]. https://easycpr-denver.com/wp-content/uploads/2015/09/Falling-TV-copy.jpg

[7]. Joseph E Shigley, Charles R Mischke, Richard G Budynas, Keith J Nisbett “Mechanical Engineering Design” – Eighth Edition Tata McGraw Hill Education Pvt., Ltd., pp 68-71.