The basic issue is one of engineering design of forks and forklifts. And i will reference counterbalance fork trucks.

As most of you may be aware, the weight on the axles of a forklift vary dramatically from empty to full. For example a Toyota 7FG25 when empty has loadins of 1470 kg on the drive and 2270 kg on the steer. When loaded the weights are 5480 kg and 750 kg.

The maximum gradeability when fully laden is 30%, and when fully loaded the force being supplied through the drive axle is sufficient to overcome the force back down the 30% slope of around 1790 kg. And this could be in theory sustained continuously.

However the tyres are capable on a good surface of achieving friction factors of 0.6 or more. So the force the drive axles in theory could supply is >= 0.6 x 5480 kg = 3290 kg or 85% greater than the force above. In pushing or pulling something some distance the drive train (gears and trasmission shaft and bearings) would be subject to much higher loads than designed and this could seriously reduce the life of the transmission.

In addition, when initially hitting the object to be pushed, inertia could result in much higher peak forces being applied to the drive train than the design loads.

Even though many forklifts are fitted with what appears to be a towing pin at the rear, this is provided to allow the forklift to be towed onto a tow truck if it has broken down et cetera.

Then there is the issue of traction on say a wet surface when the forklift is empty. The friction factor may only be 0.3 - 0.4 so in the case above the traction force available at the drive axle may be 0.3 x 1460 = 440 kg. So on a 10% slope the maximum weight of the forklift and trailer would be around 4400 kg, and as the empty forklift weighs 3730 kg the maximum trailer plus load weight would be 670 kg. And if traction is lost going up a ramp, the end result could be an uncontrollable slide back down the ramp.

If you want to use a forklift to tow a trailer you need to contact the manufacturer/ supplier and get written advice as to the limits and conditions for safe towing.

Note that when th brakes are applied hard a deceleration of 0.6 is possible so the force at the drive axle can be (5480 + 750) x 0.6 = 3740 kg BUT this force is only applied to those components from the tyre outer surface to the brake drum or disc. Generally none of the drive chain is subjected to high forces when the brakes are applied.

One final note - fork tynes are not a good device for pushing loads as expereinced drivers as well aware - the point loads can cause a lot of damage to pallets etc. I was invovled in one case where hazardous goods were placed on a truck. The forklift operator used a tyne to push 200 litre drums together to ensude they would not move in travel. The drums contained solvent. What he did not notice was that the point load cracked the drum. In travel the truck was passing a school when the leaking solvent caught fire. the resulting toxic fumes from the solvent and the other dangerous goods blew over the school and into its buildings- the end result was that every person in the school had to be decontaminated/ washed down and given full medical checks, the whole school and much of its playground and trees had to be washed down, all items in the school that could not be decontaminated had to be replaced (books and similar educational materials, carpets, curtains...). Very expensive for the company that loaded the dangerous goods, and all because a tyne was used as the pushing device.