Forklift Carriage Bounce Improvement

The goal of the project is to reduce or eliminate the bounce of the carriage on a forklift while moving over a rough or bumpy surface.

=Problem Definition= The objective of this project is to reduce the bounce in the carriage of a forklift. When a forklift drives over a bumpy surface such as a pothole, cords, hoses, and broken pallets, the carriage of the forklift tends to bounce up and down violently. This bounce is loud, and can cause the operator to spill their load if the bounce is violent enough.

=Background= The "carriage" on a forklift is the part that the forks are held by. The carriage is held up by chains that ride on a pulley on top of the lifting cylinder. The carriage is suspended in air by the lifting chains, and held vertical by the rollers on the carriage that slide up and down inside the channel in the mast. As the forklift drives over a bump, the upwards momentum of the truck will cause the carriage to lift up while the lifting cylinder does not, causing slack in the lifting chains. When the carriage falls back down, it is in free fall until all the slack in the chain is taken and the carriage is stopped abruptly by the tension in the lifting chain. To add to that, the forks are not rigidly attached to the carriage so that the width can be adjusted manually for different loads. When the forks bounce up and down on the carriage, it creates a loud, undesirable clanging sound. Hyster-Yale would like to offer a system on their forklifts as a premium option to reduce the bouncing of the carriage. We will be developing a fully-functional concept prototype that aims to reduce or totally eliminate the bounce in the carriage and the forks.

=Deliverables= We will be developing and building a fully functional prototype that will reduce the carriage bounce on the forklift. The design should be scalable to fork trucks of different lifting capacities.

=Specifications=

User Interface Requirements
There will be little to no user input required in the final design. Ideally, the mechanism will auto-activate whenever there is not an up or down command from the user controls. A disengage function should be incorporated so that the user can disengage the mechanism from the cab of the forklift.

Load Capacity
The mechanism should be able to meet the load capacity of the forklift it will be applied to. Since the goal is a scalable design, the load capacity should meet or exceed that of the parent forklift. The prototype that is being built will be mounted to a forklift with a 2.5 ton capacity.

Surface Types
The carriage damping mechanism should be effective in conditions such as bumpy gravel lots, potholes in asphalt and concrete, and a messy concrete warehouse setting.

=Design Considerations=

Hydraulic Power
If a hydraulically powered solution is pursued, there are a couple of limitations. Hydraulic pressure is limited to 2500psi working pressure. The pressure and flow rate requirement of hydraulics must not limit the abilities of other hydraulic functions on the truck. Ideally, the solution should share hydraulic power and hoses with what currently exists on the mast and carriage of the forklift. There are a limited amount of hoses that can be routed to the carriage. If separate hoses are required for the mechanism, it will limit the number of other attachments that can be implemented on the forklift.

Electrical Power
The forklift runs on a 12 Volt system. Currently, power that is routed to the carriage and mast of the forklift is limited to a headlight circuit. Electrical power faces similar complications with cable routing that hydraulic power faces. Electrical wires routed to the carriage must be able to withstand incredibly high cycle counts before fatigue. Small current draw items such as solenoids can be incorporated, while large current draw items such as motors will require more in-depth project learning.

Safety
Safety is a huge consideration in the design stage. Any components within the load line of the carriage must adhere to a factor of safety of 4. High hydraulic pressure also has the capability of breaking skin and causing fatal hydraulic injection into the bloodstream. Any hydraulic power that is implemented must be done so in a way that the operator is out of harms way in the case that a hydraulic line fails and begins spraying high-pressure oil.

=Project Learning= A few approaches were considered to dampen the bounce of the carriage. Hydraulic Accumulators, Spring/Damping systems, fork clamping mechanisms, and brakes were all researched.

Hydraulic Accumulators
Hydraulic Accumulators work as a hydraulic approach to a spring-/damper system. Pressurized hydraulic and either a pressurized gas or a spring are separated by a membrane. As Hydraulic pressure increases and the volume on the hydraulic side of the accumulator increases, the pressure exerted by the pressurized gas/spring increases, acting very similar to a spring. Accumulators are connected in parallel with the lift cylinder hydraulic circuit. Hydraulic accumulators are commonplace on forklifts with heavy attachments, such as rotators. This application is limited in its damping abilities, and are used to reduce high-impulse loads exerted on the system by a heavy empty carriage bouncing. In order to approach a solution with this route, a hydraulic accumulator would need to be developed from scratch and tuned for the weight of the empty carriage while not limiting effectiveness of the lift system on a fully loaded carriage.

Spring/Dampener System
A spring and dampener system could prove an effective approach to eliminating the bounce of the carriage. Springs and dampeners have been explored in two locations on the forklift: on the top of the lift cylinder, and in series with the lift chains. The main design consideration with this approach is that the mechanism will lie directly in the load-line of the lifting system, and must adhere to the high factor of safety requirements for any component within the lifting system. Difficulty with this approach is presented when tuning the effective spring rate and damping ratios. The weight range of the carriage can vary from a few hundred pounds up to 2 or 3 tons. Developing a spring and dampener that is effective with an empty carriage and does not affect the capabilities of a fully loaded forklift brings many challenges.

Brake System
A brake system could be incorporated into the carriage of the forklift to halt all movement of the carriage. It would effectively clamp the carriage to the mast of the forklift at all times except for when a movement is being commanded to the lift cylinder, either up or down. Design considerations here are largely spatial and hydraulic. In order to rigidly hold the carriage via a clamp on the mast, large normal forces will be required, something hydraulic power exceeds at. Routing separate hydraulic hoses to the carriage is a challenge in itself, and sharing hydraulics with the current hydraulics on the mast is desirable. A braking system must not exert an external net force, all forces must be contained within the brake "caliper" and the surface being clamped.

Force Measurement
In order to determine whether a design will work or not, quantitative measurement of the problem is required. In order to do this, strain gauges are being used to measure the forces present in the lift chains when the carriage bounces down, and a strain gauge load cell to test forces present when the carriage bounces up. Accurate strain gauge measurement takes time. While this process is being developed and executed, a faster but far less accurate method was used to measure forces. Accelerometers were placed on both the carriage and the mast of a forklift and then driven across various rough surfaces. The acceleration of the carriage relative to the mast was read and calculated, and will be used as a metric for preliminary design considerations.



Current Force Measurement Progress
The data aquired from the strain gage experiment will not be used to guide our design, and instead to validate that the design has been built strong enough. Materials for the experiment took over three months to arrive from the order date, and time has run out to make large designs guided by strain gage data.

Regardless of the long delay, data is collection is close to being completed. The gages has been adhered to the chain anchors and lead wires have been soldered. Once data acquisition code and an amplified Wheatstone bridge circuit have been completed and verified, the gages will be calibrated with varying weights. Once calibrated, the forklift will be driven over bumpy surfaces included a gravel parking lot, wood scraps, and hoses to record the inertial load that the carriage brake design will need to overcome to be effective.

=Fork Clamp= A large part of the sound and vibration that results from the carriage bounce is the forks bouncing around. The forks on the carriage has to be loosely fit to allow the operator to adjust the fit for different sized pallets or boxes. The forks are held on by hooks on the top and bottom. Our fork clamp design will implement a hydraulic cylinder on the bottom fork hook that applies a vertical force and puts tension on the fork, rigidly securing the fork in place.



=Design Process= We are currently pursuing the brake caliper design. We have been sent a carriage for a 2.5 ton forklift that our design will be mounted on once completed and machined. Our design will use a hydraulically powered clamp to apply a high normal force on the thick steel flange of the forklift mast. The friction forces present when the clamp is powered is what will resist the inertial load of the carriage bounce. This is a historical representation of our development process.

Design 1
We began with a single piston hydraulic brake welded to two long steel pins that were attached to a brake pad. The steel flange of the mast would fit between the piston and brake pad, and when activated, the piston would extend and apply a large frictional force on mast flange. The brake would be attached to the carriage by a sleeve that fits over the pins. The sleeve would allow the brake to slide on the pins and self-center when activated. This design would only apply ~1300 pounds of frictional force and would likely not keep the carriage from moving.



Design 2
After some preliminary calculations, we determined that a single cylinder would not apply enough force and more cylinders would be needed. Design 1 also does not have clearances to assemble into the forklift carriage, so the slider pins would need to be bolted in place. It will impart a large bending moment on the bolts, so spacers between the brake pad and the cylinder assembly was added so that the spacer material can share the load. detail features like oil galleys and pistons seals are also present in this model.



Design 3
The slider design proved increasingly difficult to design without breaking. Included with high-load connections and large amounts of wear over extended periods of time, the concept was replaced with a pivot. The pivot also allows for self-centering of the brake caliper while allowing for 0.25" of clearance on the front and back of the steel mast flange to reduce wear on the brake and the mast from dragging. A set of springs were implemented in this design to return the springs to their original, unclamped state so that the brake does not drag on the mast when not activated. To make room for the springs, the piston shaft diameter was reduced and a sintered bronze plate is screwed on top to hold the springs in place. Rounded contours were removed to allow for easier machining.



As this design uses bolts to hold on the brake pad, a bolt stress analysis was required to verify the strength. We were able to determine that M12x90mm bolts would be sufficient to hold the load with a load factor greater than 2. These bolts also yield an infinite lifetime under the Goodman Criteria with a safety factor of ~1.38.

=Validation= In order to validate our design, we will be building a fully functional prototype. Hyster-Yale has sent us a forklift carriage that we will be installing our completed design on. Once our work is done, the carriage will be sent back to Hyster in Portland to be installed on a functional forklift and operated on their test tracks.

=Team Members=

=Additional Documentation=

Project Schedule



Presentations