Force Required to Cut Logs

Two humanoids with a whipsaw can probably do it faster. You would be surprised until you actually try it.

Spec in your motor to be able to handle the absolute highest torque/power that it could possibly ever see. Then take this value and multiply by 1.5 (minimum) I would multiply by 2.

You will need to take into account the force required to overcome the rolling/sliding resistance of the entire system, IE the log on the rollers, the chain through the guides & sprocket, the force exerted onto the log by the blades , and so on.

It's not an easy thing to estimate with such little information. Other things you will need to take into consideration is weather or not the rest of the hydraulic system can supply the correct pressure and flow to the motor. Is it a dedicated hydraulic system (reservoir/pump/motor/valves) to pull the log? Or is it a common Power unit running various motors/cylinders?

As much as I would love to be able to spec in a motor for you which would easily yank the log through the blades it cannot be achieved without knowing much much more information.

Where are you geographically? If you are on the west coast, I could come out and take a look (I design Hydraulic systems for a living, and have power units in many mills across the globe).

This is my problem. It's not just a simple calc' to lift a certain load in a specified time. It's the indeterminable state of log and blade, however......I can say that this is a new setup and yes, the hydraulics will do other functions but this one function, I feel, will probably determine the power pack with all other functions being given access to that pressure and flow. Proportional Pressure Relief and Pressure Compensation can only go as far as the available pressure from the power pack. This is where experience matters.

I would agree within reason, that a 1.5 factor of safety would be advisable but am reluctant to go too far with this as the price has a similar multiplication factor. This is why a good guess is important. My good guess would be a motor capable of delivering 40kW at 60 rpm. Given figures like that it's a no-brainer to work out the rest but the initial guess needs to be close AND also I would need to substantiate the guess.

No, I'm not on the west coast, I'm on the east coast (of Australia) so, in land miles, we would be neighbours (Ignoring the Pacific Ocean and Coral sea).

Thanks for your input, what do you think of my guess???

Can you get any info from the saw (or blade) manufacturers? I'd be tempted to push as hard as possible without damaging the saw/blade, but be prepared to do some experimenting (in case this clogs the teeth or whatever).

Just my two-penneth.

You need to find the specs on the feed rate for the blade you are using. If each tooth of the saw blade is designed to cut say .025 of an inch. Times the number of teeth on the blade. Times the RPM of the blade. Will give you a feed rate (how fast you can move the material through the blade). This is an optimal speed your should design should allow some adjustment. As you have already stated the condition of the saw blade changes. You do not want to force the log through the blade or the cut will not be straight. If forced the blade will bend around the different density changes in the wood.

So the motor needs to move the load(log) at the feed rate. With controls to reduce feed rate.

Hi ozzb

What you say is correct BUT I have already stated a feed rate (desired by the owners) of 70m/min. Now, whether that is correct or not has little baring on the motor size as I can get velocity out of anything given enough oil flow.

What I need to know is what force the motor is needed to provide and I'm beginning to think it is almost an un-answerable question.

Tornado, you are a hero! An answer of sorts!

Well I did say in #1 what hydraulic motor to pull carriage through saw. Or at least I inferred that!

Anyway, I think you have inadvertently solved my dilemma. The twin head band saw is electrically driven so, as the feed speed increases, the power required by the saw motors will increase and NOT (so much) the power required to pull the log through. Also, there will be an optimum speed of feed set by the number of teeth on the blade and blade velocity.

I think that if I assume that your calc' involved a soft wood log and I add a bit for hard wood (perhaps x1.5) might be justifiable.

Thankfully, I only need worry my head about the hydraulics!

Nonetheless a GA in appreciation.

Some ozzie hardwood is very hard. I have seen a chain saw melt cutting Wandoo. I know you are in hydraulics but you still might need to get a handle on the species, its hardness / cutability. Whether the timber is green or recycled will also be a big factor.

I suspect the band saw blade will be about 75mm wide 6 mm thick with about 1/2 a tooth per inch ( don't you just love being Bi dimensional ). This will make it hard to equate to any hand saws but i guess you could get a relative factor by asking someone to cut through a softwood log and then one of your hardwood logs and time it.

I am only suggesting this because TORNADO gave figures for American wood and from my experience ( using West Oz hardwood ) a factor of 3 to 5 might be needed. e.g. Western Red Cedar vs Jarrah. Less for Redwood vs Vic Ash. etc.

Hi Jimrat.....cor don't I know. I melted my chainsaw on a ten year old piece of Iron Bark.

Over here in the east, I think the timber mill is processing Spotted Gum that is fairly fresh felled perhaps some Blood Wood too.

The twin head band saw will be used to cut the flitches off the main part of the trunk.

Yes, look at Tornado's calc', very bi-dimensionally BUT GOOD!

Thanks for your help!

Ah! Aurizon Yes, a PID loop is what, I'm sure, the people that deal with the wet string will use (Oh and a house brick PLC etc).

My hydraulic brain was telling me to use pressure control (of a sort yet to be determined) to provide varied pull through of the log.

Must get around to working that one out once I have sorted out all the end users!

Thanks mate!

Check (calculate) the power / torque / force to accelerate the 9.5 tons to speed in the distance required (high school Physics). I believe that you will find the peek power required is getting the carriage and log up to speed within about 4 ft, and stopping it again in that distance. Check your run out and run in distance. Varying the carriage speed while cutting will probably cause the blade to wander.

From past experience be sure to have some mechanism to capture the carriage on power failure. Using DC motors we used to use a scheme called suicide DB. The armature voltage was applied to the field and a DB grid applied across the armature on power failure. Occasionally a mill still lost the carriage out through the back wall.

Check what the current designs are using. Check with companies such as Raute or Premiere Gear what they are supplying. I believe (20 years ago) it was about 500 HP with a 150% OL capability used at each end of travel. The acceleration and deceleration times will be key to your production.

Selection of hydraulics and pressures and flow rates will also determine the sizing.

The application is brutal and is in a harsh rough and tumble environment. Build it tough for lots of abuse.

Cricky GW, you've got me worried!!!

Notwithstanding all your comments that I have noted (in blood), I think I've covered the 'carriage through the back wall' issue as that is were I anticipate siting the control cabin! If the operator does not use due care, he/she will have 9.5 tonnes on their lap.

I checked out the two companies but they seemed mainly to deal with veneer plants.

High school physics aside, I think/hope I have allowed enough for acceleration and am hoping that the chain gear and hydraulic brake can handle the deceleration (at the moment). Things may change!

Thanks for your very pertinent comments!

You seem to be missing a lot of requisite information: 1) You can probably assume a freshly sharpened saw blade. 2) Hardwood can refer to balsa or teak. Knowledge of the wood species is critical. 3) The speed of the saw blade is just as critical. 1mm/second will cut your log, but not this week. 10m/second will burn up your blade in any but the softest woods. Anything in between will be better, but still cannot be determined without more information. 4) The number of teeth in any unit length of blade as well as the kind of teeth will help determine the cutting efficiency of the saw. 5) The width of the blade will effect drag. 6) Too much force on the log could break the saw blade. My advice is to use whatever power is available to do the job and insert a clutch to reduce the feed rate when the feed resistance exceeds some high, but safe force. I cut my wood with a Stihl chain saw. The weight of the saw is an adequate feed force to cut any wood I need to cut.

Hi gene

I think that your requisite info isn't necessarily requisite. Without knowing much about your chainsaw (or mine), the force required to cut wood could easily be guessed. The motor will provide that force (that is adjustable) and velocity only comes into the equation when traversing (rather than cutting).

Clutch.......CLUTCH ????? I just can't recommend fitting a clutch in a hydraulic system!.......But I could fit a pressure limiting valve that will be set to within the limits of the blade (irrespective of the type of wood to be cut)!

A pressure limiting valve is the hydraulic equivalent of a clutch. I meant the concept of clutch, not the device used in older automobiles.

Can we come at this from a different angle? Obviously, you need sufficient FORCE to overcome frictional losses and present the end of the log to the saw blades. Thereafter, you need sufficient additional pressure to FEED the log - maintain sufficient pressure to prevent the blades chattering within the cut. Speed of cut will be determined by: Speed of sawblades, density and water content of wood, run of grain in the wood and sharpness and set of the sawblades. Too much pressure will result in : blade breakage, overheating and de-tempering of the blades or clogging with sawdust/chips. Speed of cut is determined by these - not by how hard you push it! Regards and good luck.

Hi Zadok

You have hit the crux of the matter!

Given that all your stated points relating to the saw blade are indeed true BUT in themselves, variable.

There seems to be only TWO choices:- 1) use the saw blade manufacturer's limits on applied force (good until blade becomes less than sharp, and I'm not saying 'blunt') OR 2) provide the operator with a proportional pressure control that he will have to control using 'seat of the pants' feeling.

Neither seem entirely satisfactory and there isn't any obvious constant so, unless I'm missing something, you are telling me the variables but not the solution!

My feeling is that the force albeit adjustable, will have to be constant and the operator keep a sharp eye on an amp meter measuring the load of the electric motors of the twin head band saws.

For your design you need to use the limits of the blade manufacturer plus provide about 25 to 50% more capacity for headroom. Do you have those numbers?

Can you identify the distance you have for accel and decel? We can then provide power / force required for the acceleration.

(In some mills the ends of the track are "no fly" zones.)

With the carriage weight you have you do not appear to have a homemade chainsaw mill, but rather a commercial machine, and production is key.

Do you have the dimensions of your track, carriage, accel / decel lengths, expected production rates, etc? In my experience if the blade manufacturers say x fpm at y lbs force, those numbers will be tested at commissioning, be normal production within the week, and then pushed beyond in the next months as the limits for acceptable production quality and tool life is balanced out.

(As a point of reference on a veneer lathe with 9 ft logs, up to 30 inch diameter max, with a diet of 10 inch blocks, peeling to a three inch core, the cycle is completed typically in 6 seconds, ie 10 logs per minute. Surface velocity is 1200 fpm, accel time is 0.5 to 0.7 seconds. Motor accelerates up to 2200rpm. peeling force varies between 30 to 60 lb per inch. Peek motor power is used for accel and decel. typical motor is 300 Hp with a field range 850 to 2200 rpm. If the base speed is increased to 1200 or 1300 rpm typically 500Hp is used. The lathe is a low inertia machine with less than 200 lb ft^2 total)

Not my specialty, but the thoughts are that if the saw is cutting properly, the inertia and running friction of the table+log is predictable from previous advice. The effort onto the blades can be derived from the blade suppliers, plus the relevant safety factor for this to be "robust" in operation.

To monitor the blade performance, measure the deflection (strain) in the bandsaw head as that will directly relate to the transferred force from the blades and provide a direct relationship to the pressure of the wood against the blade.

From what you have described however, it would seem that the mill owner has already determined the capability of their saw to cope with that feed rate. Properly set teeth should clear the flitch so there should be minimal drag from the blade.

East Coast you say. How far North (or South) of Newcastle is the mill? I know an "old hand" who was involved in building mills in the '80's who may still remembers the numbers.

Talked with my "old" friend and he indicated the challenge is to get "force per tooth engaged" and since your timber diameter is variable this is not constant.

suggestion was to determine ability of saw to remove swarf for largest log diameter and then all smaller diameters will be handled.

He also cautioned about feed rates for different timbers. Spotted gum is very different from bloodwood and tallow-wood is different again.

He did STRONGLY suggest the co-efficient of friction numbers need to be re-visited. The rails are unlikely to be "clean steel on steel" and will get a covering of sap and dust.

My input is "If you're going for hydraulic control, why no run constant pressure instead of constant velocity?"

Thanks for that JaE'

Funny how all things change so quickly. After a phone call with the client this morning, they now want to go electric for this function and keep the hydraulics for clamping etc.

This sort of suits me as I can now get on with it.

Thanks to you and all other contributers for the info that was gratefully received.

Do let the forum know whether or not <...70 meters/min...> was achieved, and with what rating of motor, as it may be helpful to someone else on the forum.

...and on what species of timber and what size the cuts are, of course.

The Hydraulic system should be a proportional feedback type system.

I realize this is an old post, we have a Meadows Mill with a 60” circular saw, number of insert teeth I don’t have right now.

comming off the farm, we have it powered of a 85 HP tractor. Softwood it’s sufficient. Hardwood it will bear down on the engine. We’d use 100HP for hard wood

normally established saw mills the saw isn’t power by a hydraulic, but electric and I’d double the HP. Saw blades are hammered to a certain RPM, you don’t want the blade RPM dropping.you heat the blade, and the blade will wander and get tapered boards.