Recumbent Tricycle Brake - Research Paper Example

Aim To design brakes of recumbent tricycle for disabled user Information Major force used in braking of bicycle (recumbent tricycle is frictional force. Following are the main types of brakes. Rim type brakes Disc type brakes Drum type brakes Rim brakes In this case braking force is applied by friction pads to the rim of cycle. This generates torque in opposite direction thus slowing down the bicycle. Disc brakes A special disc is attached to the wheel hub of bicycle which rotates with wheel. Callipers are attached to the frame or fork .Callipers consists of brake pads and piston. When brakes are applied pads are squeezed on the disc thus slowing down the bicycle. These are more powerful than rim brakes. Drum Brakes Drum brakes have become obsolete because they are heavier and more complicated, In this case both pads are pressed outward against the braking surface on the inside of the hub's shell, which is packed with grease. Selection of brake of recumbent tricycle We have selected Disc type of brakes for the above project. As these are more powerful than the remaining two. As braking power is high this will be of great help for disabled person. They also perform better in water compared to rim brakes. Disk brakes are able to operate at a higher mechanical advantage than rim brakes because disc rotors in good condition are more true than rims in good condition, and as such do not need to retract as far from the rim when released. We are also providing these brakes to both rear wheels. This is due to fact that if the brakes are applied to the front wheel the force will cause the bicycle to tumble because of more braking power. Various parts that are to be designed for brakes are listed below. 1) Single actuating calipers 2) Calliper housing 3) Brake pads 4) Disc 5) Brake levers 6) Mechanical cable Design & principle of working Wheel Braking Force B On disc at angle A Reaction Force R D C The first basic problem (Annan,James) of a disk brake is that the position of the disk calliper means that the frictional force of the brake pads on the disk acts largely to push the wheel downwards, broadly in the direction of the open fork ends. The force that the pads exert on the disk is very large - much much greater than the force from a rim brake (due to the smaller diameter of a disk compared to wheel rim) and it is also at an inappropriate angle. The relevant forces and dimensions on the wheel are indicated. We have the braking force B and the ground reaction force R acting at the contact patch. The disk calliper exerts a force D as indicated, tangential to the disc at the point of calliper contact. The radii of the disk and wheel are r1 and r2, and finally the angle of the dropout exit is 'A' in front of vertical. For this fork and calliper, the force D is virtually vertical. If it wasn't, there would be another angle 'b' for the angle the disk force makes behind the vertical. Let's assume we have a bike + rider weighing 90kg in total, The rear wheel applied brakes give decelerating effect of 0.1g (3m/s^2).With the rear brake alone (this is a reasonable estimate for braking). The rearward force is 90 x 0.1 x g = 90N, and the vertical reaction force is 90 x g =900N (all the weight is on the rear wheel). Taking moments around the axle, the force D exerted by the disk is given by D = 270 x r2 / r1. Let us take radius of wheel to be 330 mm(r2).Let us assume r1= 72.5mm is the effective radius of the disk (ie to the centre of the 'force at the pads, rather than the outer edge = 10mm less than the full radius of 165mm / 2). So D = 410N.This force is acting on both rear wheels. So total force =410 +410 =820 N, acting vertically downwards. This acts entirely on the left hand side of the wheel, but the ground reaction force and braking forces are split equally between the two sides. And also the reaction force is on both wheels. So we are left with resultant forces of 820 - 900/4 = 595N vertically down and 540/4 = 135N rearward. The sum of those is equivalent to a single force of 610N acting downwards at an angle of 8 degrees behind vertical. Resolving this parallel to the dropout opening angle (= 18 degrees ahead of vertical for a head tube angle of 72 degrees) leaves a force of 610N x cos(18+8) = 547.8N out to slow down the wheel. The most recent ISO standard (1996) specifies that the quick release should be able to withstand a direct pull of 2300N, symmetrically applied, without slipping. This is relatively big number compared to 547.8 N Single actuating calipers Brake caliper is assembly of piston and brake pads. The pistons are made of light materials like aluminum. Calipers are mainly of 2 types. Fixed caliper Floating caliper Fixed calliper:- In this case caliper does not move with respect to disc. These type of calipers are more expensive and complicated. Floating caliper:- In this case calliper moves with respect to disc. When brakes are applied a piston on one side of the disc pushes the inner brake pad until it makes contact with the braking surface, then pulls the caliper body with the outer brake pad so pressure is applied to both sides of the disc. For our tricycle we select floating type of caliper. In our case we will choose Rear Disc Mount .The disc mount on the back of your bike should be on the down slope of the rear triangle (just above the quick release or axle). In this is the case bike will have 2 small brackets welded onto the down slope of the rear triangle. This will allow to mount the disc calliper directly onto your frame. The top part of this adaptor then has holes in the same position as would be found on an international standard frame. Calliper housing Calliper housing are usually of cast iron and used to protect caliper from outside dirt, dust. For simplicity of manufacturing we will choose simple design as given below Calliper housing Piston with brake pad Disc Brake blocks and pads There are 2 brake pads per caliper. So for 2 rear wheels total four pads are used.Brake pads are made of asbestos /ceramics. This is done for high friction with brake pad material. With brake pad material embedded in the disc in the process of bedding while wearing evenly. The pads work with a very thin layer of their own material and generate a semi-liquid friction boundary that creates the actual braking . Precaution The brake pads must usually be replaced regularly. Disc As per IS ( International standard) we can use disc of 160 mm, 185mm, or 203mm in diameter. We will choose 185 mm disc .Disc have circular holes on them to dissipate the heat generated by braking. As the speed of recumbent bicycle is not much we will provide a simple 5 mm dia holes on circular pitch of disc. Brake levers These are provided near the handle to facilitate easy braking. They are used to transalate the mechanical advantage into more movement of calliper piston Mechanical (cable) Standard pull levers are used in our case. They work well with calliper brakes .We will be using centre pull cantilever system as they give assured performance. Manufacturing Following points are to be remembered while manufacturing above disc brakes. Calliper housing should be machined out of cast iron bar. Disc are made of crome plated steel with outside dia 185mm and inside dia of 150mm. Circular holes of 5mm are provided on pitch of 165 mm dia . Brake pad material is made of any soft metal like ceramics/asbestos. Calliper piston is made of aluminium Brake lever and mechanical cable length will be according to size of recumbent bicycle. Risks Brakes fail due to sticking failure, which can occur due to dirt or corrosion entering at least one mounting mechanism and stopping its normal movement. This can cause the pad attached to the caliper to rub on the disc when the brake is not engaged, or cause it to engage at an angle. Sticking can occur due to infrequent vehicle use, failure of a seal or rubber protection boot allowing debris entry, dry-out of the grease in the mounting mechanism and subsequent moisture incursion leading to corrosion, or some combination of these factors. Friction-induced heat warping of the disc may also occur. For this reason brakes should be checked regularly for there function. Necessary action should be initiated Refrences 1) IS standard for brakes 2) Annan, James. "Disk brakes and quick releases - what you need to know" 3) Bicycle Glossary from Sheldon Brown's website Read More