Shear strenght test device: Design of a device for testing shear strenght on winter roads

Shear strenght test device

Design of a device for testing shear strenght on winter roads

Gustav Fält

Automotive Engineering, bachelor's level 2020

A ^BSTRACT

When buying a new car today customers expect that the safety systems built in the car and its tires will do its job in every condition. This is especially important when driving on winter roads due to the decrease in friction between the tire and the road surface. To get further understanding how snow behaves on winter roads, knowing how the shear strength in the hard-packed snow found on winter roads changes when doing for example multiple brake test in the same tracks can be of great importance when designing a new, safer product. This thesis will go through the design process of a new device designed to measure shear strength in winter test tracks. The device consists of an electric motor powered by 12 or 24 Vdc connected to a worm gear style gearbox and can measure up to 200 Nm of torque

S AMMANFATTNING

När man köper en ny bil idag förväntar sig kunden att däcken och säkerhetssystemen i bilen gör sitt jobb i alla förhållanden. Det är speciellt viktigt när man kör på vintervägar på grund av den minskade friktionen mellan däck och underlag. För att öka förståelsen hur snö beter sig på just vintervägar, att veta hur skjuvstyrkan i snön ändras efter till exempel multipla bromstester i samma hjulspår kan vara mycket viktig information när man designar en ny och säkrare produkt. Denna rapport kommer gå igenom designprocessen för en ny testrigg för att testa just skjuvstyrkan i hårdpackad snö.

Testriggen består av en elmotor som drivs av 12/24 Vdc som är sammankopplad till en växellåda av typen snäckväxel och kan mäta ett vridmoment upp till 200 Nm.

C ^ONTENT

Abstract ... 2

Sammanfattning ... 3

List of figures ... 5

Introduction ... 6

Background ... 6

Purpose ... 6

Boundaries ... 6

Theory ... 7

Snow ... 7

Definition of shear strength ... 7

Method ... 8

Design process ... 8

Device requirements ... 8

Bevameter device ... 9

Results ... 10

Components ... 10

Torque sensor ... 10

Motor and gearbox ... 11

Adapterplate ... 12

Shear head ... 13

Bracket ... 15

Clamps ... 16

Completed shear device ... 21

Discussion... 23

Future work ... 23

Appendix A. Drawings ... 24

References ... 30

L IST OF FIGURES

Figure 1. Basic idea for a bevameter device ... 9

Figure 2. HBM TB1A 200Nm torque sensor ... 10

Figure 3. Transcenco ECMM250/030/063 ... 11

Figure 4. Adapterplate ... 12

Figure 5. Anders Bodins shear head used for soft snow ... 13

Figure 6. Shear head with flanges ... 14

Figure 7. Shear head with a rubber ring ... 14

Figure 8. Bracket to mount the gearbox/engine to ... 15

Figure 9. Clamp that are welded to the frame ... 16

Figure 10. Small clamp ... 16

Figure 11. Vertical frame... 17

Figure 12. Bottom frame ... 18

Figure 13. Right side brace ... 18

Figure 14. Left side brace ... 19

Figure 15. Bottom tray ... 19

Figure 16. Assembled frame ... 20

Figure 17. Assembled shear device ... 22

I NTRODUCTION

B

If you want to conduct tire and vehicle test in snow and ice conditions with consistent results the friction between the tire and the road might not be enough information to get viable results. You might also need to know the shear strength in the snow you are conducting your tests on. Due to a gap in science and research of shear strength in hard packed snow this thesis will go through the design process of a device designed to test shear strength in hard packed snow in a lab environment.

P

The purpose of this thesis is to design a device used to measure the shear strength in hard packed snow found in vehicle test tracks. This to further get consistent results when doing tire and vehicle system testing on winter roads and thus help companies develop safer products for its customers.

B

The device ought to be simple with consideration to construction and design. It shall also be designed to work in a laboratory environment

T ^HEORY

S

Newly fallen snow on a winter road has a low density of <100𝑘𝑔𝑚⁻³, the snow is a mixture of solid ice crystals, liquid water and air [3]. This changes fast on a winter road when vehicles pack it while driving on it and weather changes the grain structure of the snow. The snow ends up at around 500𝑘𝑔𝑚⁻³ on an average winter road [4]. Due to a gap in science there isn’t much research on shear strength in hard packed snow. Shear strength in snow cover in alpine environments however is well investigated using shear frames, shear vanes and laboratory shear apparatus [7] [8] [9] [10] [11]

[12]. These investigators concluded that shear strength correlates significantly with snow density and less significantly with field observations of snow temperature and grain size [13]

D

Shear strength is a term used in soil mechanics to describe how much shear stress a soil can withstand. Basically, shear strength in snow can be idealized by having two components, cohesion and friction [14]. Shear strength is measured in Pascal (Pa).

M ^ETHOD

D

This project started of with researching if there is any modern equipment to measure shear strength in hard packed snow in a lab environment. Existing equipment for measuring shear strength in hard packed snow is extremely limited, there is however some for testing softer snow. E.g. Tsutomu Nakamuras vibrating apparatus used in lab environment [5]. Seeing there was a huge gap in this field I decided to look at equipment for testing shear strength in alpine environments and get inspiration from that. What I came up with was two different kind of devices. The first one was a shear frame device based on Sommerfields 0,025𝑚² shear frame used in alpine environments [6]. The other one was a wingdrill device based on a vane shear test device. Shortly after, I came in contact with Joonas Mahonen who is doing his doctoral thesis on shear and compression strength in snow for the snowmobile market at BRP Lynx and Luleå Technological University. Joonas already had a concept based on a bevameter device and I ended up basing my device on his. The frame was the first thing to be designed since it could be designed without knowing the exact components for the gearbox, motor and torque sensor. When me and Joonas had decided what engine, gearbox and torque sensor that would fit our requirements the rest of the device could be designed. All parts were designed using Siemens CAD software NX 12.

Device requirements

• Be able to measure accurately up to 200Nm of torque

• Withstand a lateral force of 1000N

• Run on 12/24Vdc

• Be able to rotate the shear head at low speeds, 10-12rpm

B

“The Bevameter technique is based on two vertical plate penetration tests with different sizes of the sinkage plates to obtain the size dependency and two shear tests at different normal pressure to obtain the cohesion and the friction angle of the Mohr-Coulomb equation. The shear tests are performed with shear rings, one to determine the internal shearing using grousers and one smooth ring covered with rubber to obtain the rubber-snow shearing relationship.” [15]

The device designed in this paper will not include any penetration test but will only focus on shearing. See the basic idea of a bevameter device in figure 1. The shear stress-shear displacement relationship which is used to describe both internal and external shearing often used for fresh snow proposed by Janosi and Hanamoto [16] which is shown in Eq. (1)

𝑠 = (1 − 𝑒^−𝐾𝑗) (𝑐 + 𝑝𝑡𝑎𝑛𝜃) (𝑘𝑃𝑎) (1)

In this equation s are the shear stress (kPa), j are the shear displacement (cm), p is the normal pressure (kPa) and K (cm), c (kPa) and θ (°) are terrain parameters derived from experimental data.

The last parenthesis is the maximum shear stress according to the Mohr-Coulomb equation.

Usually a bevameter is powered by hydraulics but this device is fully electric. This due to simplicity in designing and ease of adapting the whole device for field use later. The device is powered by 12 or 24 Vdc which is found in all cars on the market. It can easily be adapted for field use simply by building a new frame to hold the motor/gearbox and sensor assembly.

The engine and gearbox assembly are mounted to a torque sensor which is mounted to a bracket that can slide up and down the frame. The shear head is then mounted to the gearbox with a splined shaft. To get enough lateral force so the shear head wont slip, barbell weights can be attached to the horizontal rods attached to the bracket. A snow sample is placed on the tray below the shear head.

Figure 1. Basic idea for a bevameter device

Results

C

Due to the fact this project is in collaboration with Joonas Mahonen who is doing his doctoral thesis at Lynx and Luleå university of technology this rig uses the same motor/gearbox and torque sensor as he will use in his device for testing shear strength in loose snow aimed towards the snowmobile industry.

Torque sensor

The chosen torque sensor is the HBM TB1A 200 Nm (figure 2). This sensor was chosen due to its compact size, easy mounting and it fitted the requirements of the lateral force and torsional force.

The HBM TB1A is available in seven nominal maximum torque capacities (100 Nm, 200 Nm, 500 Nm, 1 kNm, 2 kNm, 5 kNm, and 10 kNm).

Figure 2. HBM TB1A 200Nm torque sensor

Motor and gearbox

The motor/gearbox combination chosen to power the rig is the Transcenco ECMM250/030/063 (figure 3) which is a permanent magnet DC motor connected to two worm gear drives in a die cast aluminum housing. This combination can deliver 173 Nm at 10 rpm which is enough torque for this application. The engine shown in figure 3 will not be mounted in this orientation due to fitment issues, instead it will be mounted twisted anticlockwise 90°. The engine delivers 250 W and is powered by 12/24 Vdc.

Figure 3. Transcenco ECMM250/030/063

Adapterplate

To mount the torque sensor to the gearbox an adapter is needed (figure 4). This will be milled out of solid steel.

Figure 4. Adapterplate

Shear head

Three different shear heads were designed. One is Anders Bodins shear head [2] which is designed for soft snow (figure 5). The other ones are a bit smaller with 100 mm in diameter. One is similar to Anders shear head with flanges on the bottom (figure 6). This one is designed to determine the internal shearing. The other one doesn’t have flanges but uses a smooth rubber ring instead (figure 7). This one is designed to determine the rubber-snow shearing.

Figure 5. Anders Bodins shear head used for soft snow

Figure 6. Shear head with flanges

Bracket

The bracket (figure 8) to mount the gearbox/motor assembly to the frame is constructed in 8 mm sheet metal with two horizontal rods welded to the sides of. These rods are meant for attaching dumbbell weights to achieve desired lateral force between the shear head and the snow sample so the shear head wont slip in the snow sample. They can also be used when adjusting the height of the shear head simply by grabbing the rods and lift the bracket with the gearbox/motor mounted to it.

Figure 8. Bracket to mount the gearbox/engine to

Clamps

Two different kind of clamps are used to mount the bracket to the frame. Four longer ones (figure 9) are welded on the vertical frame, these together with four smaller ones (figure 10) hold the 30 mm pipes to the vertical frame. Eight small ones are then used to hold the bracket to the 30 mm pipes.

Figure 9. Clamp that are welded to the frame

Frame

The frame is constructed with 30x30 mm square tubing for easy manufacturing. The vertical frame (figure 11) is welded together and so is the bottom frame (figure 12). The two parts are then bolted together, and two diagonal braces (figure 13 and 14) made of 20 mm steel tubing is also bolted between the two parts for added stability. A tray for adding snow samples is then bolted the bottom frame (figure 15). Four of the longer clamps are welded to the vertical frame to hold two 30 mm tubes. The tubes are held in place with four of the smaller clamps. These tubes enable the shear head to move 500 mm vertically for easy placement of the snow sample. The assembled frame is shown in figure 16.

Figure 11. Vertical frame

Figure 12. Bottom frame

Figure 13. Right side brace

Figure 14. Left side brace

Figure 15. Bottom tray

Figure 16. Assembled frame

C

To complete the device the torque sensor is bolted to the adapterplate. The engine/gearbox are then bolted together with the torque sensor and adapterplate. This assembly is then bolted to the bracket. To mount the bracket to the 30 mm pipes, eight of the smaller clamps are used. The shear head is then attached to the gearbox via the splined output shaft. The completed shear device is shown in figure 17.

The shear device is operated by following orders

1. Loosen the bolts holding the bracket to the tubes if fastened.

2. Lift the bracket to get enough room under the shear head to place a snow sample on to the tray. Snow can either be collected from the road piece you are interested in testing, or you could use a tool to compress snow to the desired density.

3. Lock the bracket in place by tightening the bolts holding the bracket to the tubes.

4. Place the sample on the tray.

5. Loosen the bolts and lower the bracket until the shear head is touching the sample.

6. Add dumbbell weights to the horizontal rods on the bracket to get the desired lateral force so the shear head wont slip when running the device. The shear head should be pressed down 2 cm in the snow.

7. Run the device until the shear head has rotated 40° or 10 cm when using the bigger shear head designed for soft snow. This ends up as a 0.67 sec run time at 10 rpm. These are guidelines and might be needed to change once testing has begun.

8. After the test is done, remove the weights and do steps 5-2 in reversed order.

Figure 17. Assembled shear device

D ISCUSSION

Due to the gap in science in this field there was problems finding enough information to design everything supported by data. There is plenty of research done on shear strength in soft snow in alpine terrain, but this cannot be translated directly to hard packed snow. For example, Eq. (1) which might not work for this application. Only testing will show that. Therefor the smaller 100 mm shear head might need to change size and shape depending how it performs in hard packed snow once testing begins. Because of the tight time frame of just 10 weeks the device was never constructed. This mostly because of long delivery times of all the component and the fact that the design process took longer than expected. I am still very happy with the result and think it will help Joonas with his research and help fill the gap in science once it has been built and testing has begun.

F

Since building this device didn’t fit the timeframe, the next step for this project is building it.

Since all the drawings are done but aren’t checked that needs to be done first. After all drawings have been checked the parts can then be manufactured. The assembling of the device will be quick because of the simplicity in design. The electronics is something that must be figured out as well. This should be simple by connecting the torque sensor to an ADC (analog to digital converter) and a 12 Vdc car battery and a controller for the engine. Next season when the snow starts falling you could start the testing phase of the project. Results from these tests can then be used to increase knowledge of how winter test tracks get affected by multiple brake tests in the same tracks for example. That in turn can help companies develop safer product for their customers which will result in fewer accidents in winter traffic.

A ^PPENDIX A. D ^RAWINGS

R EFERENCES

[1] Bevameter - Development of a device to characterize the characteristics of soft terrain, Anders Bodin, (2002), Luleå University of Technology, Sweden

[2] Characterization of snow strength properties for oversnow vehicle mobility, Anders Bodin and Iwan Wasterlund, (2002), Luleå University of Technology, Sweden, Swedish University of Agricultural Sciences, Sweden

[3] Measurement of mechanical properties of snow for simulation of skiing, Martin Mössner, Gerhard Innerhofer, Kurt schindelwig, Peter Kaps, Herwig Schretter, Werner Nachbauer

[4] Processing snow for high strength roads and runways, Renee M. Lang a, George L. Blaisdell b, Christian D'Urso c, Gregory Reinemer c, Mark Lesher d.

[5] A dynamic method to measure the shear strength of snow, Tsutomu Nakamura, Osamu Abe, Ryuhei Hashimoto, Takeshi Ohta

[6] Instructions for using the 250 cm2 shear frame to evaluate the strength of a buried snow surface, Sommerfeld, R., (1984). USDA Forest Service Research Note RM-446

[7] Snow mechanics with references to soil mechanics, Bader et all Der Schnee und seine Metamorphose, Haefeli, R. (1939), Beitrge zur Geologic der Schweiz, Geotechnische Serie, Hydrologic, Lieferung 3, Bern. (U.S.A. SIPRE Translation No. 14, 1954.)

[8] Some physical properties of alpine snow, Research Report 271, Keeler, C.M. (1969) U.S. Army Cold Regions Research and Engineering Laboratory, Hanover, New Hampshire.

[9] Some mechanical properties of alpine snow, Keeler, C.M. and Weeks, W.F. (1967),Montana, 1964- 66, Research Report 227, U.S. Army Cold Regions Research and Engineering Laboratory, Hanover, New Hampshire.

[10] Les variations de la resistance de la neige, International Symposium on Scientific Aspects of Snow and Ice Avalanches, Roch, A. (1966), International Association of Hydrological Sciences Publication No. 69, pp. 86-99.

[11], Mekhanicheskie svoystva snega (Mechanical properties of snow), Voitkovsky, K.F. (1977) Nauka, Moscow, 126 pp.

[12], Avalanche defence mechanics, McClung, D.M. (1974) Ph.D. Thesis, University of Washington, Seattle, Washington, 103 pp.

[13] The shear strength index of alpine snow, R. Perla, National Hydrology Research Institute, Ottawa, Ontario (Canada) T.M.H. Beck Alberta Forest Service, Calgary, Alberta (Canada) and T.T.

[16] The analytical determination of drawbar pull as a function of slip for tracked vehicles in deformable soils, Janosi, Z. and Hanamoto, B., (1961) Proc. Of the 1 st International Conference on the Mechanics of Soil-Vehicle Systems, Edizioni Minerva Tecnica, Torino, Italy