MINA: PLAN DE MINADO

User

Producer

Client

Reseller

39 In order to be able to design a lifting aid for heavy objects it is crucial to understand what forces are allowed to be transmitted to the human body during use. Therefore an analysis is made to determine the best way for lifting heavy objects and how much this object is allowed to weigh. In addition to being safe and healthy, lifting with the device should also be comfortable. An effort is therefore made to find guidelines that specify how much stress on a particular body part is allowed before the user experiences discomfort.

Lifting heavy objects

Guidelines for heavy lifting have been around for a while and are aimed to reduce the risk of musculoskeletal disorders. Of these tools the NIOSH lifting equation is the most comprehensive. The equation around which the method is build can be used to calculate the maximum allowable weight and gives a score that indicates how much risk an average healthy employee has on developing an injury.

The equation is based on a lifting constant of 23 kg, which represents the maximum amount of weight a person is allowed to lift under ideal circumstances (Middlesworth, n.d.). This number is multiplied with a number of weighing coefficients (M) that can be determined by measuring the aspects listed below (Figure 39).

LC (51) x HM x VM x DM x AM x FM x CM = RWL

 H = Horizontal location of the object relative to the body

 V = Vertical location of the object relative to the floor

 D = Distance the object is moved vertically

 A = Asymmetry angle or twisting requirement

 F = Frequency and duration of lifting activity

 C = Coupling or quality of the workers grip on the object

The recommended weight limit (RWL) that comes out of the equation can be converted in the lifting index (LI) by dividing it by the 23. A number larger than 1, signals that the average employee runs the risk of developing an injury, while a number smaller than 1 means that the job can be accomplished without any health hazards. A quick analysis whether furniture movers abide the NIOSH lifting equation shows however that this is not the case in all situations. Table

4, which lists the average weights of a number of household items, shows that some objects are too heavy even when lifted with two persons. This discrepancy has already led to a high number of health issues amongst professional furniture movers. Silverstein and Adams (2007) for example found that furniture moving companies belong to the top 5 of sectors with the most applications for worker’s compensation due to work related musculoskeletal disorders of the neck, back and upper extremities.

Design guidelines for comfortable lifting

Comfort is not only determined by the amount of weight that is lifted but, also by how the force is transmitted onto the body. Although there are no stringent rules about this, a number of design guidelines should be kept in mind.

When designing an ergonomic handle for an application that requires a firm grip a number of design guidelines need to be followed. These where abstracted from a checklist developed by Patkin (2001).

The length of the handle should be at least 10-15 centimetres to fit the width of the palm. The thickness of the handle has to be such that the thumb can just cover the

Table 4 - Average weight of furniture and appliances (FRN, 2009). Bookcase 21 Large desk 27 Dining table 29 3-seater sofa 42 Fridge-freezer 51 Double wardrobe 55 Large cabinet 77

Figure 39 - Variables for calculating RWL (Middlesworth, n.d.).

Figure 40 - Ergonomic handle (“Backyard Gardening”).

Figure 41 - Ergonomic lifting harness (“Grip System”).

index and middle fingers. For adult males this means the handle should have a diameter of around 3-4 cm. The shape of the handle can be slightly flattened and thickened in the middle to prevent sliding. To increase grip further the handle should have smooth grooves that fit the fingers (Figure 40). Sharp edges and protruding shapes should be avoided to prevent stress hotspots. Surface roughness may help to prevent sliding but should be not be overdone.

In addition to handles for lifting, chapter 3.2 has shown that harnesses can be used to transfer forces to areas of the body that can handle more stress. Figure 41 shows such a harness in more detail. This particular harness has been designed by a team of scientists from the University of California to reduce neck, shoulders and back injuries sustained by heavy lifting. The harness features padded straps and supports that transfer the lifting force evenly over the shoulder and the lower back. According to Paskiewicz and Fathallah (2007) this reduces the probability of developing lower back disorder by about 40%.

A number of things have been learned from the work done in this chapter. The stakeholder analysis has provided insight in the requirements that the user, client and reseller will demand form the suction adhesion device. What catches the eye when looking at this first set of requirements, is the importance of the device’s ability to communicate the status of the adhesion process. This improves trust in the device’s capabilities and makes the user more comfortable with lifting big and expensive items. In section 3.2, market research showed what competing solutions are already available. It can be concluded that there are a couple of low-tech solutions that cost less than €50,-, but also a lot of high-tech devices that require an investment of thousands of euros. In between these two segments however no solutions could be found. It is therefore a good idea to design a device that fills this hole in the market.

The analysis of the use environment yielded important parameters that act as boundaries in between which the suction adhesion device has to function. It can be concluded that all substrates have more or less a similar amount of surface free energy. This value usually lies between 30-50 mJm-2. Typical roughness on the other hand varies a lot more. Glass is by far the smoothest with a roughness of only 0.0006 µm, while the roughness of plaster can be as high as 400 µm.

The ergonomics surrounding heavy lifting have revealed a couple of interesting things. People working in the furniture industry have to frequently lift items that are heavier than what is considered safe. This leads to health issues by overloading the neck, shoulder and lower back. The ergonomic solutions found for this problem generally do not reduce the amount of force, but transmit it to the body in a better way. In this way the NIOSH limit, which assumes that items are lifted by hand, can be safely exceeded. Especially the use of lifting harnesses seems to be promising in order to reduce lifting related health issues.

All requirements that were encountered during the analysis of the problem have been concluded in the table on the next page. This list of requirements is used as a guideline for the design of the suction adhesion device and as a checklist to validate whether it meets all the demands.

41 Table 5 - List of requirements.

Analysis section Requirement Specifications

1. 1. The suction adhesion device mimics natural suction adhesion systems The suction adhesion device uses the 12 suction adhesion strategies. 3.1 and 3.4 2. The suction adhesion device significantly lowers the effort needed to lift difficult

to handle objects.

Attachment and detachment within 5 seconds and no straps required. Provides stable attachment point. Abides to NIOSH lifting guideline and design guidelines for comfortable lifting.

3. The suction adhesion device is easy and intuitive to operate. Maximum of 5 buttons and no double functions.

4. The suction adhesion device communicates the remaining adhesion time. Using an interface that specifies the amount of adhesion time left in minutes. 5. The suction adhesion device communicates the adhesion strength. Using an interface that specifies the adhesion strength in kg.

6. The suction adhesion device warns the user when it is about to let go. Using clear audible and visible warning signals.

3.1 and 3.4 7. The suction adhesion device is mobile and easy to handle. The device is cordless and has a maximum weight of 2.3 kg.

8. The suction adhesion device is rugged and reliable. The device survives drop from 2 meters, uses high quality components and is not susceptible to wear.

9. The suction adhesion device does not require a lot of maintenance. No lubricants or sealants that need to be refreshed and easy to clean. 10. The suction adhesion device does not damage or contaminate the handled

objects.

Does not leave behind any residue and distributes the adhesion force over the attachment area.

11. The suction adhesion device has a modular platform with fixed interfaces and uses standardized components when possible.

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12. The suction adhesion device has an attractive appearance that fits the preferences of the target group.

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3.1 and 3.2 13. The suction adhesion device should be affordable for the target group. Retail price should be between €150,- and €800,-.

3.2 and 3.4 14. The suction adhesion device has to deliver a competitive level of performance. The suction adhesion device has to be able to produce an isotropic adhesion force that is enough to lift 80 kg.

3.3 15. The suction adhesion device is able to adhere under all use circumstances. Can adhere to surfaces with surface free energies between 30 and 169 mJm-2, surface roughness between 0.0006 and 400 µm and maximum permeance of 1.67 μmPa-1s-1. Can deal with water, grease and dust. Functions under temperatures between -6 and 40 °C, humidity up to 49.8 g of water per kg of air and atmospheric pressures between 80 and 106 kPa.

In this chapter a design is created that fits the list of requirements that was developed in chapter 3. In order to determine the best design direction, first a number of conceptual designs are created. These are evaluated and the best design is elaborated into the final concept.

The conceptual designs are a result of an investigation that determines the functional structure of the suction adhesion device and the keydrivers behind the device. Together with the biomimicry strategies from chapter 1 these ideas are formed into three concepts. The concepts each tackle the problem from a different angle but use the same functional structure.

The concepts are rated based on the keydrivers, and the best concept is chosen for further elaboration. A large part of this elaboration consists out of the design and characterisation of the suction cup. A model is created that predicts the pulling resistances of the suction cup. In addition to this the pressure-drop underneath the suction cup and the sealing capabilities of the sealing rim are determined.

Although the suction cup design takes up a large portion of this chapter also effort is put into developing the system behind it. An impression of the final product is generated by determining what components are used, how they look like and what method is used to make or obtain them. This information is subsequently used to give a rough approximation of the costs to produce such a device. Also a proposal of a business plan around the suction cup device is presented to show that it can be made according to the requirements of the new circular economy paradigm.

43 To get a deeper understanding of what the list of requirements asks from the suction adhesion device they are converted into keydrivers. These are the motives that lie behind the requirements and are a more abstract representation of what the stakeholders want from the device. Having a set of keydrivers makes it easier to create and evaluate concepts that have not yet been fully developed. This is because many requirements cannot be tested until after the system has been fully specified.

Total costs of ownership (TCO)

The total costs of ownership are an important keydriver that comprises all the monetary costs the owner has to make to buy and use the device. To make the device an interesting option for the potential buyers the TCO should be as low as possible. However other stakeholder, like the manufacturer and reseller will want to maximize profit by increasing the TCO. Therefore it is better to strive for a high ratio between the total benefits of ownership and the total costs of ownership.

Examples of costs that are associated with the ownership of the suction adhesion device are listed below.